remove references to observations_root_dir (dir_obs)

[mathause]

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

Another minor cleanup (after #267)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e1711b8) 87.57% compared to head (df7f45b) 87.56%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #324      +/-   ##
==========================================
- Coverage   87.57%   87.56%   -0.01%     
==========================================
  Files          38       38              
  Lines        1674     1673       -1     
==========================================
- Hits         1466     1465       -1     
  Misses        208      208              

Flag	Coverage Δ
unittests	87.56% <100.00%> (-0.01%)	⬇️

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

Files	Coverage Δ
mesmer/calibrate_mesmer/calibrate_mesmer.py	92.50% <ø> (-0.07%)	⬇️
mesmer/calibrate_mesmer/train_gt.py	91.54% <ø> (ø)
mesmer/calibrate_mesmer/train_gv.py	95.00% <ø> (ø)
mesmer/calibrate_mesmer/train_lt.py	88.23% <ø> (ø)
mesmer/calibrate_mesmer/train_lv.py	84.33% <ø> (ø)
mesmer/core/computation.py	100.00% <100.00%> (ø)
mesmer/core/grid.py	97.82% <ø> (ø)
mesmer/core/utils.py	100.00% <100.00%> (ø)
mesmer/core/weighted.py	100.00% <ø> (ø)
mesmer/create_emulations/create_emus_gv.py	89.18% <ø> (ø)
... and 6 more

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