Fixes cell specific sample generation

[ssenan]

• Resolves cell specific sample generation not working for hESCT0 #193
• Updates cell specific sampling code to simplify device allocation and introduces metrics that allow us to measure cell specific samples motif composition with heat maps
• Removes old cell specific heat map generation function
• Moves the checkpoint folder out of the library
• provides tests for functions related to validating the function of code updates: compare_motif_list and one_hot_encode

[codecov]

Codecov Report

Merging #195 (0dd592e) into main (2743a65) will increase coverage by 6.85%.
The diff coverage is 84.21%.

@@            Coverage Diff             @@
##             main     #195      +/-   ##
==========================================
+ Coverage   17.65%   24.50%   +6.85%     
==========================================
  Files          14       16       +2     
  Lines         895      910      +15     
  Branches      104      103       -1     
==========================================
+ Hits          158      223      +65     
+ Misses        736      684      -52     
- Partials        1        3       +2     

Files Changed	Coverage Δ
src/dnadiffusion/utils/sample_util.py	43.85% <0.00%> (+0.75%)	⬆️
src/dnadiffusion/utils/train_util.py	0.00% <0.00%> (ø)
src/dnadiffusion/metrics/metrics.py	39.47% <100.00%> (+39.47%)	⬆️
tests/test_metrics.py	100.00% <100.00%> (ø)
tests/test_utils.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes