Reject unsupported PolarShellDistribution dimensions

[BJMCox]

Fixes #554.

Validate that the base distribution is two-dimensional at construction. Previously accepted 1D and 3D inputs now fail early with ArgumentError.

Main code: +2/-0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.45%. Comparing base (6e8795f) to head (25d41cd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #562   +/-   ##
=======================================
  Coverage   65.45%   65.45%           
=======================================
  Files         121      121           
  Lines        7060     7061    +1     
=======================================
+ Hits         4621     4622    +1     
  Misses       2439     2439           

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