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, improve PathMover class hierarchy
• use numpy.bytes_ instead of numpy.string_
• move density collection and correction to shooting point selection
• aimmd.distributed: tqdm progress bars for transition path sampling

[codecov]

Codecov Report

❌ Patch coverage is 35.20782% with 265 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.71%. Comparing base (8cabc33) to head (d4d15ad).

Files with missing lines	Patch %	Lines
aimmd/distributed/pathmovers.py	30.20%	104 Missing ⚠️
aimmd/base/density_collection.py	25.37%	50 Missing ⚠️
aimmd/distributed/pathsampling.py	15.21%	39 Missing ⚠️
aimmd/distributed/spselectors.py	32.07%	36 Missing ⚠️
aimmd/distributed/dataclasses.py	65.71%	23 Missing and 1 partial ⚠️
aimmd/base/storage.py	35.71%	9 Missing ⚠️
aimmd/distributed/committors.py	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   36.68%   37.71%   +1.03%     
==========================================
  Files          43       45       +2     
  Lines        5237     5160      -77     
  Branches      815      791      -24     
==========================================
+ Hits         1921     1946      +25     
+ Misses       3205     3102     -103     
- Partials      111      112       +1     

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