Curve Fit Optimisation and R-Squared

[alexdaniel654]

Proposed changes

Moved the curve fitting used for exponential fitting of T1/T2/T2* maps into its own sub-module. Fixed the issue with multiprocessing not scaling over large images (#165). In the process added R-Squared values to fit outputs (#198).

Checklists

• [x] I have read and followed the CONTRIBUTING document
• [x] This pull request is from and to the dev branch
• [x] I have added necessary documentation (if appropriate)
• [x] I have updated documentation which becomes obsolete after my changes (if appropriate)
• [x] Files added follow the repository structure (if appropriate)

[codecov[bot]]

Codecov Report

Merging #205 (8e722b0) into dev (cfcd84d) will increase coverage by 0.13%. The diff coverage is 97.97%.

@@            Coverage Diff             @@
##              dev     #205      +/-   ##
==========================================
+ Coverage   97.62%   97.75%   +0.13%     
==========================================
  Files          41       45       +4     
  Lines        3581     3744     +163     
==========================================
+ Hits         3496     3660     +164     
+ Misses         85       84       -1     

Impacted Files	Coverage Δ
ukat/mapping/fitting/relaxation.py	93.44% <93.44%> (ø)
ukat/mapping/diffusion.py	97.12% <94.11%> (-0.81%)	:arrow_down:
ukat/mapping/fitting/tests/test_relaxation.py	98.13% <98.13%> (ø)
ukat/mapping/t1.py	99.37% <98.64%> (+0.95%)	:arrow_up:
ukat/mapping/t2star.py	96.85% <98.87%> (+1.91%)	:arrow_up:
ukat/mapping/fitting/__init__.py	100.00% <100.00%> (ø)
ukat/mapping/fitting/tests/__init__.py	100.00% <100.00%> (ø)
ukat/mapping/t2.py	99.01% <100.00%> (+2.42%)	:arrow_up:
ukat/mapping/tests/test_diffusion.py	98.29% <100.00%> (+0.06%)	:arrow_up:
ukat/mapping/tests/test_t1.py	99.17% <100.00%> (+0.05%)	:arrow_up:
... and 2 more

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