Use 1 point for degenerate income distributions

[Mv77]

This PR modifies the engines for transitory and permanent income shocks so that if the distribution is degenerate (sigma == 0.0) they use only one point to approximate it. This is useful, for instance, in retirement.

Currently, with TranShkCount and PermShkCount being constant, multiple points are being used even if sigma==0.0. This behavior was probably my own making, in my recent updates.

• 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.

[alanlujan91]

YES! This is a great PR @Mv77! Was just talking to @iworld1991 about how I always have to remember to set PermShkCount = 1 when a random process is shut down.

This can also help when comparing RiskyAsset solutions when we shut down uncertainty.

[codecov-commenter]

Codecov Report

Merging #1115 (731e9a9) into master (ea73795) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1115   +/-   ##
=======================================
  Coverage   73.83%   73.83%           
=======================================
  Files          69       69           
  Lines       10636    10636           
=======================================
  Hits         7853     7853           
  Misses       2783     2783           

Impacted Files	Coverage Δ
HARK/ConsumptionSaving/ConsIndShockModel.py	85.82% <100.00%> (ø)

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[Mv77]

Ha! Thanks for the endorsement!

[sbenthall]

Review discussion on Feb. 23 2022 : It's a little odd how this interacts with the baked-in constant income for retirement, but as it stands this PR is just fine and should be merged.