Refactoring and cleanup, remove unnecessary code and reduce duplication

[hejung]

• renamed 'CommittorEngineSpec' to 'MDEngineSpec' and moved it to a more central file to facilitate reuse in shooting PathMovers
• use 'MDEngineSpec' as input for all shooting PathMovers
• use numpy.bytes_ instead of numpy.string_

[codecov]

Codecov Report

❌ Patch coverage is 38.25137% with 113 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.12%. Comparing base (8cabc33) to head (454afa1).

Files with missing lines	Patch %	Lines
aimmd/distributed/pathmovers.py	32.00%	102 Missing ⚠️
aimmd/distributed/dataclasses.py	66.66%	6 Missing ⚠️
aimmd/distributed/committors.py	62.50%	3 Missing ⚠️
aimmd/base/storage.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   36.68%   37.12%   +0.44%     
==========================================
  Files          43       44       +1     
  Lines        5237     5201      -36     
  Branches      815      804      -11     
==========================================
+ Hits         1921     1931      +10     
+ Misses       3205     3159      -46     
  Partials      111      111              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.