Refactor utils

[mj-will]

nessai.utils was getting rather long so I decided to refactor it into a submodule whilst writing the missing tests.

In this PR:

• Refactor nessai.utils
• Add tests
• Tweak determine_rescaled_bounds and detect_edges. The default behaviour remains the same but some edge cases now work as expected.

[codecov-commenter]

Codecov Report

Merging #96 (169ceca) into main (a56fdbf) will increase coverage by 2.19%.
The diff coverage is 99.34%.

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
+ Coverage   92.11%   94.30%   +2.19%     
==========================================
  Files          29       39      +10     
  Lines        3475     3513      +38     
==========================================
+ Hits         3201     3313     +112     
+ Misses        274      200      -74     

Flag	Coverage Δ
unittests	94.30% <99.34%> (+2.19%)	⬆️

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Impacted Files	Coverage Δ
nessai/reparameterisations.py	89.96% <ø> (ø)
nessai/flowsampler.py	38.88% <50.00%> (ø)
nessai/flowmodel.py	93.28% <100.00%> (ø)
nessai/utils/__init__.py	100.00% <100.00%> (ø)
nessai/utils/distance.py	100.00% <100.00%> (ø)
nessai/utils/distributions.py	100.00% <100.00%> (ø)
nessai/utils/hist.py	100.00% <100.00%> (ø)
nessai/utils/indices.py	100.00% <100.00%> (ø)
nessai/utils/io.py	100.00% <100.00%> (ø)
nessai/utils/logging.py	100.00% <100.00%> (ø)
... and 15 more

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