move v0 code to own folder

[mathause]

This illustrates how we could move all code from v0 to it's own folder without disrupting the usage of the project. This is not something I want to do right now but wanted to have here for reference. Obviously the other folders would need to be moved as well.

Another option would be to keep packages = find:, add legacy_mesmer/__init__.py, add from legacy_mesmer import calibrate_mesmer to mesmer/__init__.py. However, from mesmer.calibrate_mesmer import train_gt will then fail.

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

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -1.95 ⚠️

Comparison is base (79c495c) 86.24% compared to head (4410eb7) 84.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #262      +/-   ##
==========================================
- Coverage   86.24%   84.29%   -1.95%     
==========================================
  Files          38       31       -7     
  Lines        1512     1089     -423     
==========================================
- Hits         1304      918     -386     
+ Misses        208      171      -37     

Flag	Coverage Δ
unittests	84.29% <ø> (-1.95%)	⬇️

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see 7 files with indirect coverage changes

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[mathause]

This will happen after v0.9.0 and before v1.0.0 (have to decide if in v0.10.0 or in v0.11.0)