Functional Solvers

[alanlujan91]

This PR contains some suggestions for @mnwhite's ongoing solver overhaul.

Simple functions that can be re-used within a "family" of models, but should not be expected to "jump" out of their own file.

[codecov]

Codecov Report

Attention: Patch coverage is 53.23383% with 188 lines in your changes are missing coverage. Please review.

Project coverage is 70.56%. Comparing base (1d707c7) to head (fb74f68).
Report is 1 commits behind head on master.

Files	Patch %	Lines
HARK/ConsumptionSaving/ConsPortfolioModel.py	1.81%	162 Missing ⚠️
HARK/ConsumptionSaving/ConsIndShockModel.py	89.02%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1395      +/-   ##
==========================================
- Coverage   71.72%   70.56%   -1.16%     
==========================================
  Files          84       84              
  Lines       13910    14305     +395     
==========================================
+ Hits         9977    10095     +118     
- Misses       3933     4210     +277     

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[alanlujan91]

@mnwhite updated MPCmaxUnc to apply to MPC_for_interpolation and MPCmaxNow to be the effective mpc max

[alanlujan91]

@mnwhite added docstrings and merging as discussed