Combine labeled distributions

[Mv77]

Augments the existing combine_indep_dstns to take and use labeled distributions. If all the inputs are of the class DiscreteDistributionLabeled, the result will be another labeled discrete distribution with the labels of the variables in the original distributions.

Addresses #1190

• Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• Updated documentation of features that add new functionality.
• Update CHANGELOG.md with major/minor changes.

[codecov-commenter]

Codecov Report

Base: 73.56% // Head: 73.62% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (7814af1) compared to base (cfaaf25).
Patch coverage: 96.96% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1191      +/-   ##
==========================================
+ Coverage   73.56%   73.62%   +0.06%     
==========================================
  Files          72       72              
  Lines       11941    11973      +32     
==========================================
+ Hits         8784     8815      +31     
- Misses       3157     3158       +1     

Impacted Files	Coverage Δ
HARK/distribution.py	82.80% <93.75%> (+0.32%)	⬆️
HARK/tests/test_distribution.py	100.00% <100.00%> (ø)

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