Don't hide categories when creating the RooMinimizer

Indeed, the RooMinimizer can't deal with floating category variables.

But instead of hiding the categories when creating the RooMinimizer, one
can also set them to constant and the minimizer will not complain too.

This solution is less hacky than hooking into `getParameters()` of the
CachingSimNLL.

This was originally introduced by @gpetrucc in
#574

Quoting from that PR:

```
For the dirty flag propagation to the NLL when the categories are
changed, and option to avoid reporting the categories in getParameters()
to avoid complaints from RooMinimizer
```

So indeed, hiding the categories was supposed to address these warnings,
which are also gone when the categories are just set to constant.

Author: guitargeek

interface/CachingNLL.h

@@ -182,7 +182,6 @@ class CachingSimNLL  : public RooAbsReal {
         static void forceUnoptimizedConstraints() { optimizeContraints_ = false; }
         void setChannelMasks(RooArgList const& args);
         void setAnalyticBarlowBeeston(bool flag);
-        void setHideRooCategories(bool flag) { hideRooCategories_ = flag; }
         void setHideConstants(bool flag) { hideConstants_ = flag; }
         void setMaskConstraints(bool flag) ;
         void setMaskNonDiscreteChannels(bool mask) ;
@@ -195,7 +194,6 @@ class CachingSimNLL  : public RooAbsReal {
         const RooAbsData  *dataOriginal_;
         const RooArgSet   *nuis_;
         RooSetProxy        params_, catParams_;
-        bool hideRooCategories_ = false;
         bool hideConstants_ = false;
         RooArgSet piecesForCloning_;
         std::unique_ptr<RooSimultaneous>  factorizedPdf_;

src/CachingNLL.cc

@@ -838,7 +838,6 @@ cacheutils::CachingSimNLL::CachingSimNLL(const CachingSimNLL &other, const char
     nuis_(other.nuis_),
     params_("params","parameters",this),
     catParams_("catParams","Category parameters",this),
-    hideRooCategories_(other.hideRooCategories_),
     hideConstants_(other.hideConstants_),
     internalMasks_(other.internalMasks_),
     maskConstraints_(other.maskConstraints_),
@@ -1240,10 +1239,10 @@ cacheutils::CachingSimNLL::getParameters(const RooArgSet* depList, Bool_t stripD
     RooArgSet *ret;
     if (internalMasks_.empty()) {
         ret = new RooArgSet(params_); 
-        if (!hideRooCategories_) ret->add(catParams_);
+        ret->add(catParams_);
     } else {
         ret = new RooArgSet(activeParameters_); 
-        if (!hideRooCategories_) ret->add(activeCatParameters_);
+        ret->add(activeCatParameters_);
     }
     if (hideConstants_) RooStats::RemoveConstantParameters(ret);
     return ret;
@@ -1255,10 +1254,10 @@ bool cacheutils::CachingSimNLL::getParameters(const RooArgSet* depList,
 {
     if (internalMasks_.empty()) {
         outputSet.add(params_);
-        if (!hideRooCategories_) outputSet.add(catParams_);
+        outputSet.add(catParams_);
     } else {
         outputSet.add(activeParameters_);
-        if (!hideRooCategories_) outputSet.add(activeCatParameters_);
+        outputSet.add(activeCatParameters_);
     }
     if (hideConstants_) RooStats::RemoveConstantParameters(&outputSet);
     return true;

src/CascadeMinimizer.cc

@@ -14,6 +14,7 @@
 #include <RooStats/RooStatsUtils.h>
 
 #include <iomanip>
+#include <memory>
 
 boost::program_options::options_description CascadeMinimizer::options_("Cascade Minimizer options");
 std::vector<CascadeMinimizer::Algo> CascadeMinimizer::fallbacks_;
@@ -53,11 +54,23 @@ CascadeMinimizer::CascadeMinimizer(RooAbsReal &nll, Mode mode, RooRealVar *poi)
 }
 
 void CascadeMinimizer::remakeMinimizer() {
-    cacheutils::CachingSimNLL *simnll = dynamic_cast<cacheutils::CachingSimNLL *>(&nll_);
-    if (simnll) simnll->setHideRooCategories(true);
-    minimizer_.reset(); // avoid two copies in memory
-    minimizer_.reset(new RooMinimizer(nll_));
-    if (simnll) simnll->setHideRooCategories(false);
+  std::unique_ptr<RooArgSet> nllParams(nll_.getParameters((const RooArgSet*)nullptr));
+  RooArgSet changedSet;
+
+  // The RooMinimizer can't deal with floating categories, so we need to set them constant at least while creating the minimizer.
+  for (RooAbsArg *arg : *nllParams) {
+    auto *cat = dynamic_cast<RooCategory *>(arg);
+    if (cat && !cat->isConstant()) {
+      cat->setConstant(true);
+      changedSet.add(*cat);
+    }
+  }
+
+  minimizer_.reset();  // avoid two copies in memory
+  minimizer_ = std::make_unique<RooMinimizer>(nll_);
+
+  // Change categories back to how they were
+  utils::setAllConstant(changedSet, /*constant=*/false);
 }
 
 bool CascadeMinimizer::freezeDiscParams(const bool freeze)