Purge NPSS from mission.jl

[askprash]

Purging NPSS from mission.jl. Plan is to purge it completely but retain/provide some useful utility tools that users can use to plug TASOPT to NPSS (or other propsys models in the future).

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (0f30cb3) 59.73% compared to head (991c011) 58.83%.
Report is 2 commits behind head on main.

❗ Current head 991c011 differs from pull request most recent head ca08927. Consider uploading reports for the commit ca08927 to get more accurate results

Files	Patch %	Lines
src/mission/mission.jl	98.62%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
- Coverage   59.73%   58.83%   -0.91%     
==========================================
  Files          63       63              
  Lines       12091    12240     +149     
==========================================
- Hits         7223     7201      -22     
- Misses       4868     5039     +171     

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