Make CoopCompKRG a true SMT surrogate model (#808)

This PR moves `CoopCompKRG` from `applications` to `surrogate_models`. 

To respect the `SurrogateModel` API:
* add a `comp_var` option to specify the [nx, ncomp] mapping or set the
`n_comp` option which allows to compute `comp_var` randomly. This allows
to make `train()` call compatible.
* support derivatives and variance_derivatives (implementations
inherited from `KrgBased`)

Resolves #804

Author: relf

doc/_src_docs/applications.rst

@@ -20,7 +20,6 @@ Three methods are available:
    applications/mfkplsk
    applications/ego
    applications/podi
-   applications/cckrg
 
 
 The intent is to provide applications of surrogate models in higher level methods.

doc/_src_docs/applications.rstx

@@ -9,6 +9,7 @@ SMT implements several surrogate models related to Gaussian process regression:
 * `GEKPLS` leverages on derivatives training data to improve the surrogate model quality.
 * `MGP` takes into account the uncertainty of the hyperparameters defined as a density function.   
 * `SGP` implements sparse methods allowing to deal with large training dataset as others implementations have a time complexity of :math:`O(n^3)` as well as a :math:`O(n^2)` memory cost in the number :math:`n` of training points.
+* `CCKRG` implements cooperative components Kriging, a way of fitting a high-dimensional ordinary Kriging model by sequential lower-dimensional component model fits. For each component, only the associated hyperparameters are optimized. All other hyperparameters are set to a so-called cooperative context vector, which contains the current best hyperparameter values.
 
 Here below, the links to the dedicated pages:  
 
@@ -23,3 +24,5 @@ Here below, the links to the dedicated pages:
    gpr/gekpls
    gpr/mgp
    gpr/sgp
+   gpr/cckrg
+

doc/_src_docs/surrogate_models/gpr.rst

@@ -17,9 +17,9 @@ References
 Usage
 -----
 
-.. embed-test-print :: smt.applications.tests.test_cckrg , TestCCKRG , run_cckrg_example
+.. embed-test-print :: smt.surrogate_models.tests.test_cckrg , TestCCKRG , run_cckrg_example
 
 Options
 -------
 
-.. embed-options-table :: smt.applications , CoopCompKRG , options
+.. embed-options-table :: smt.surrogate_models , CoopCompKRG , options

doc/_src_docs/surrogate_models/gpr.rstx

@@ -8,7 +8,6 @@
 from .moe import MOE, MOESurrogateModel
 from .vfm import VFM
 from .podi import PODI, SubspacesInterpolation
-from .cckrg import CoopCompKRG
 from .mixed_integer import (
     MixedIntegerSamplingMethod,
     MixedIntegerSurrogateModel,
@@ -31,7 +30,6 @@
     "Evaluator",
     "PODI",
     "SubspacesInterpolation",
-    "CoopCompKRG",
     "MixedIntegerSamplingMethod",
     "MixedIntegerSurrogateModel",
     "MixedIntegerKrigingModel",