ENH: Validate base data objects' attributes at instantiation

[jhlegarreta]

Validate base data objects' attributes at instantiation: ensures that the attributes are present and match the expected dimensionalities.

[jhlegarreta]

Cross-ref #332 (comment).

Notes: slightly different convention and naming wrt 3686a5e:

• Suggest private functions.
• Name functions as *validator instead of *validation (i.e. actually taking action). *validation looks better for a function name, *validator would be better for a class name.

So, cosmetic changes to be discussed.

[codecov]

Codecov Report

❌ Patch coverage is 96.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.16%. Comparing base (3f684e1) to head (d3d57fc).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nifreeze/data/base.py	96.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #335      +/-   ##
==========================================
+ Coverage   77.57%   78.16%   +0.58%     
==========================================
  Files          28       28              
  Lines        1824     1882      +58     
  Branches      191      203      +12     
==========================================
+ Hits         1415     1471      +56     
- Misses        344      346       +2     
  Partials       65       65              

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