Add a unit to all fields

[HealthyPear]

This PR adds a unit to all fields, in particular

• the default unit has been set to None
• new utility function to retrive subblock units as dictionaries
• new dtype builder for data subblocks
• added unit tests for all but run and event header, which depend on the version Corsika

This is both useful by itself and a precursor for #14.

[codecov]

Codecov Report

Base: 96.19% // Head: 95.79% // Decreases project coverage by -0.39% ⚠️

Coverage data is based on head (77c08f4) compared to base (6d7096d).
Patch coverage: 92.59% of modified lines in pull request are covered.

❗ Current head 77c08f4 differs from pull request most recent head 274371e. Consider uploading reports for the commit 274371e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   96.19%   95.79%   -0.40%     
==========================================
  Files          19       20       +1     
  Lines         394      452      +58     
==========================================
+ Hits          379      433      +54     
- Misses         15       19       +4     

Impacted Files	Coverage Δ
corsikaio/subblocks/run_header.py	84.00% <75.00%> (-3.50%)	⬇️
corsikaio/subblocks/event_header.py	88.57% <76.92%> (-3.43%)	⬇️
corsikaio/subblocks/__init__.py	100.00% <100.00%> (ø)
corsikaio/subblocks/data.py	100.00% <100.00%> (ø)
corsikaio/subblocks/dtypes.py	100.00% <100.00%> (ø)
corsikaio/subblocks/event_end.py	100.00% <100.00%> (ø)
corsikaio/subblocks/longitudinal.py	100.00% <100.00%> (ø)
corsikaio/subblocks/run_end.py	100.00% <100.00%> (ø)
tests/test_units.py	100.00% <100.00%> (ø)

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[maxnoe]

Minor comment, otherwise a very nice addition, thanks!