GP algebra docstring

[nw13slx]

• add docstrings
• clean up a bit for no_cpus in gp_algebra and dft_interface
• clean up the _par functions in gp_algebra

[codecov-io]

Codecov Report

Merging #120 into master will increase coverage by 1.56%.
The diff coverage is 84.83%.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
+ Coverage    54.6%   56.16%   +1.56%     
==========================================
  Files          31       31              
  Lines        5000     4994       -6     
==========================================
+ Hits         2730     2805      +75     
+ Misses       2270     2189      -81

Impacted Files	Coverage Δ
flare/output.py	95.28% <ø> (ø)	⬆️
flare/mc_simple.py	21.36% <ø> (+9.25%)	⬆️
flare/gp_algebra.py	87.84% <63.41%> (+21.54%)	⬆️
flare/dft_interface/cp2k_util.py	90.4% <80%> (+0.15%)	⬆️
flare/gp.py	89.39% <89.39%> (-0.42%)	⬇️

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