Suggested changes for Hierarchical EM implementation

[jcapriot]

Summary

Implements my suggestions in #1296.

PR Checklist

• If this is a work in progress PR, set as a Draft PR
• Linted my code according to the style guides.
• Added tests to verify changes to the code.
• Added necessary documentation to any new functions/classes following the
  expect style.
• Marked as ready for review (if this is was a draft PR), and converted
  to a Pull Request
• Tagged @simpeg/simpeg-developers when ready for review.

What does this implement/fix?

• Rename to Hierarchical simulations
• Simplifies code for the hierarchical simulations (and fields) by making use of inheritance.

Additional information

Also adapted some relevant tests to pytest parameterized formats.

[codecov]

Codecov Report

Attention: Patch coverage is 39.28571% with 51 lines in your changes are missing coverage. Please review.

Project coverage is 68.56%. Comparing base (b85f758) to head (afe5e63).
Report is 50 commits behind head on EMsimulationFaceProperties.

❗ Current head afe5e63 differs from pull request most recent head 39244bc. Consider uploading reports for the commit 39244bc to get more accurate results

Files	Patch %	Lines
SimPEG/base/pde_simulation.py	27.86%	44 Missing ⚠️
SimPEG/electromagnetics/frequency_domain/fields.py	50.00%	4 Missing ⚠️
SimPEG/electromagnetics/utils/testing_utils.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                       @@
##           EMsimulationFaceProperties    #1411       +/-   ##
===============================================================
- Coverage                       82.54%   68.56%   -13.99%     
===============================================================
  Files                             167      167               
  Lines                           25700    26095      +395     
===============================================================
- Hits                            21214    17891     -3323     
- Misses                           4486     8204     +3718     

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