multiple index space grid operator apply

[sslattery]

This pull request adds a version of apply() to the GridOperator which will thread over a user-specified set of index spaces. It uses the multiple index space version of Cajita::grid_parallel_for to accomplish this and therefore passes the user an additional index indicating which index space is currently active. This will be very useful for implementing boundary conditions or custom loop structures over subsets of the domain.

[codecov-commenter]

Codecov Report

Merging #35 (695560a) into master (752bebb) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
+ Coverage   92.95%   93.10%   +0.14%     
==========================================
  Files          43       43              
  Lines        4129     4175      +46     
==========================================
+ Hits         3838     3887      +49     
+ Misses        291      288       -3     

Impacted Files	Coverage Δ
src/Picasso_GridOperator.hpp	100.00% <100.00%> (ø)
unit_test/tstGridOperator2d.hpp	98.64% <100.00%> (+2.28%)	⬆️
unit_test/tstGridOperator3d.hpp	99.02% <100.00%> (+2.43%)	⬆️

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[terwin]

It would be nice to have a test that uses multiple index spaces.