fix(nupdown): only set two_fermi when nupdown is non-zero

When nupdown=0.0 is explicitly set in INPUT, it should behave the same
as not setting nupdown at all. Previously, reading any nupdown value
would unconditionally set two_fermi=true, causing different code paths
even when nupdown=0.0.

Now two_fermi is only set to true when nupdown != 0.0, making nupdown=0
equivalent to not specifying the parameter.

Author: dyzheng

source/source_io/module_parameter/read_input_item_elec_stru.cpp

@@ -297,7 +297,7 @@ Then the user has to correct the input file and restart the calculation.)";
         item.availability = "";
         item.read_value = [](const Input_Item& item, Parameter& para) {
             para.input.nupdown = doublevalue;
-            para.sys.two_fermi = true;
+            para.sys.two_fermi = (doublevalue != 0.0);
         };
         item.reset_value = [](const Input_Item&, Parameter& para) {
             if (para.input.nspin == 1)