train_gt: use separate_hist_future

[mathause]

• Closes #xxx
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

Refactor train_gt to use separate_hist_future. This simplifies and de-duplicates the code. It still looks a bit convoluted, though.

[codecov]

Codecov Report

Patch coverage: 91.30% and project coverage change: -0.05% ⚠️

Comparison is base (fc43f37) 86.37% compared to head (3fa20e6) 86.33%.

❗ Current head 3fa20e6 differs from pull request most recent head 56d59f5. Consider uploading reports for the commit 56d59f5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #281      +/-   ##
==========================================
- Coverage   86.37%   86.33%   -0.05%     
==========================================
  Files          38       38              
  Lines        1549     1537      -12     
==========================================
- Hits         1338     1327      -11     
+ Misses        211      210       -1     

Flag	Coverage Δ
unittests	86.33% <91.30%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
mesmer/calibrate_mesmer/train_gt.py	91.30% <89.47%> (-0.16%)	⬇️
mesmer/utils/convert.py	95.83% <100.00%> (+0.18%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.