Fix local

[mbkumar]

This PR changes fix_all to local_fix_all and adds a replacement fix_all that fixes global DOFs.

[codecov]

Codecov Report

Merging #222 (166ea85) into master (079c4db) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #222   +/-   ##
=======================================
  Coverage   90.82%   90.83%           
=======================================
  Files          59       59           
  Lines        7725     7733    +8     
=======================================
+ Hits         7016     7024    +8     
  Misses        709      709           

Flag	Coverage Δ
unittests	90.83% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/simsopt/_core/optimizable.py	90.10% <100.00%> (+0.14%)	⬆️
src/simsopt/mhd/profiles.py	98.59% <100.00%> (ø)

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[landreman]

Excellent.

In the readthedocs tutorials (example_coils.rst, example_islands.rst, example_quasisymmetry.rst, example_vmec_only.rst), can we use fix_all() instead of local_fix_all() and use unfix_all() instead of local_unfix_all()? The shorter function names are more intuitive and work equally well here I think.

I took the liberty of adding a bit to the docs page optimizable.rst about the new functions. Feel free to edit.