Class for risky-asset consumers

[Mv77]

The only model with a risky asset to store savings currently in HARK is ConsPortfolioModel. However, there are various additional models of this type that are works in progress:

• My own two-asset model here.
• A portfolio model that takes housing into account. @alanlujan91, @llorracc and I are working on it.

It would be useful to have a class representing agents with risky assets, from which ConsPortfolio and these two works-in-progress could inherit. This PR implements such class by taking the general methods from ConsPortfolio, and makes ConsPortfolio inherit from it.

• 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

Merging #1012 (13cd47d) into master (6d157ea) will decrease coverage by 0.00%.
The diff coverage is 75.90%.

@@            Coverage Diff             @@
##           master    #1012      +/-   ##
==========================================
- Coverage   71.67%   71.66%   -0.01%     
==========================================
  Files          63       64       +1     
  Lines        9197     9234      +37     
==========================================
+ Hits         6592     6618      +26     
- Misses       2605     2616      +11     

Impacted Files	Coverage Δ
HARK/ConsumptionSaving/ConsRiskyAssetModel.py	74.02% <74.02%> (ø)
HARK/ConsumptionSaving/ConsPortfolioModel.py	66.08% <100.00%> (-1.20%)	⬇️
HARK/ConsumptionSaving/ConsIndShockModelFast.py	51.05% <0.00%> (ø)

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