Errors for 1d models

[mj-will]

This PR adds explicit checks to catch cases where the model is one-dimensional or a one-dimensional input is being passed to the normalising flow.

[codecov-commenter]

Codecov Report

Merging #121 (ef23b95) into main (67fe6e3) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
+ Coverage   94.94%   95.00%   +0.05%     
==========================================
  Files          39       39              
  Lines        3583     3626      +43     
==========================================
+ Hits         3402     3445      +43     
  Misses        181      181              

Flag	Coverage Δ
unittests	95.00% <100.00%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
nessai/flows/nsf.py	97.05% <100.00%> (+0.18%)	⬆️
nessai/flows/realnvp.py	100.00% <100.00%> (ø)
nessai/model.py	100.00% <100.00%> (ø)
nessai/reparameterisations.py	91.34% <100.00%> (+0.03%)	⬆️

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