Fix the normal mode displacement TS check

[alongd]

The normal model displacement check was fixed, a new module for this check was added.
Tests added, though more thorough testing for the overall functionality are still needed.

[codecov]

Codecov Report

Attention: Patch coverage is 94.43609% with 37 lines in your changes missing coverage. Please review.

Project coverage is 74.41%. Comparing base (fb16f4a) to head (06254f9).

Files with missing lines	Patch %	Lines
arc/checks/nmd.py	89.11%	13 Missing and 8 partials ⚠️
arc/species/converter.py	82.75%	5 Missing ⚠️
arc/checks/ts.py	62.50%	1 Missing and 2 partials ⚠️
arc/species/species.py	75.00%	3 Missing ⚠️
arc/checks/nmd_test.py	99.25%	1 Missing and 1 partial ⚠️
arc/reaction.py	96.66%	0 Missing and 2 partials ⚠️
arc/scheduler.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #768      +/-   ##
==========================================
+ Coverage   74.09%   74.41%   +0.31%     
==========================================
  Files         101      103       +2     
  Lines       28005    28465     +460     
  Branches     5860     5935      +75     
==========================================
+ Hits        20751    21182     +431     
- Misses       5786     5810      +24     
- Partials     1468     1473       +5     

Flag	Coverage Δ
	?
unittests	74.41% <94.43%> (+0.31%)	⬆️

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