to_dict() and from_dict() functionality for Coregionalize Kernel and MixedNoise Likelihood class, appveyor CI resurrected (#951)

This PR adds two main things to GPy:
- to- and from-dict functions for the kernels listed belop
- a fix for the appveyor CI
Please see the squashed commit messages listed below.
Authors: @gehbiszumeis @ppk42 respectively
Reviewer: @ekalosak 

---
* new: added to_dict() method to Coregionalize kernel class

* new: added to_dict() method to MixedNoise likelihood class

* fix: made Y_metadata dict content serializable

* fix: typo

* added additional needed parameters to to_dict() method for Coregionalize kernel + added _build_from_input dict method

* new: added possibility to build MixedNoise likelihood from input_dict

* Y_metadata conversion from serializable to np.array when loading from dict

* fix: rework Y_metadata part for compatibility with unittests !minor

* conda cleanup in appveyors pipeline

* conda clean up after conda update

* conda clean before conda update

* try pinning packages for conda

* revert all conda changes

* conda clean all (not only packages)

* use conda update anaconda

* pin conda package

* pin conda package

* try installing charset-normalizer beforehand

* try to get from conda-forge

* revert all conda changes

* Try to fix the conda update challange.

See: https://community.intel.com/t5/Intel-Distribution-for-Python/Conda-update-Conda-fails/td-p/1126174

It is just a try for a different context/(conda version).

* Still fixing build error on appveyor

I also use a newer miniconda version for greater python versions.

* Update appveyor.yml

Thinking it over it decided to use miniconda38 for all python versions unless python 3.5.

* revert miniconda versioning changes

* adjust GPy version in appveyor.yml

* 1st attempt bring the appveyor build to life again

* #955 fixing ci build on appveyor

After bringing the miniconda env to work again, the wrong matplotlib version was used. This commit should fix that.

* #955 Fix CI build

Freezing numpy and scipy was a bad idea.
I freeze matplotlib  dependend  on the python version only.

* add: built_from_dict method for White Kernel

Co-authored-by: Peter Paul Kiefer <ppk42@users.noreply.github.com>
Co-authored-by: Peter Paul Kiefer <dafisppk@gmail.com>

setup.py

@@ -118,8 +118,10 @@ except ModuleNotFoundError:
     ext_mods = []
 
 install_requirements = ['numpy>=1.7', 'six', 'paramz>=0.9.0', 'cython>=0.29']
+matplotlib_version = 'matplotlib==3.3.4'
 if sys.version_info < (3, 6):
     install_requirements += ['scipy>=1.3.0,<1.5.0']
+    matplotlib_version = 'matplotlib==3.0.0'
 else:
     install_requirements += ['scipy>=1.3.0']
 
@@ -174,7 +176,8 @@ setup(name = 'GPy',
                         'optional':['mpi4py',
                                     'ipython>=4.0.0',
                                     ],
-                        'plotting':['matplotlib >= 3.0',
+                        #matplotlib Version see github issue #955
+                        'plotting':[matplotlib_version,
                                     'plotly >= 1.8.6'],
                         'notebook':['jupyter_client >= 4.0.6',
                                     'ipywidgets >= 4.0.3',