Fix ES solver

[streeve]

Also adds to CI

[codecov]

Codecov Report

Merging #48 (09f97ec) into master (3d22919) will increase coverage by 5.8%.
The diff coverage is 93.1%.

@@           Coverage Diff            @@
##           master     #48     +/-   ##
========================================
+ Coverage    78.9%   84.7%   +5.8%     
========================================
  Files          12      12             
  Lines         801     881     +80     
========================================
+ Hits          632     747    +115     
+ Misses        169     134     -35     

Impacted Files	Coverage Δ
example/example.cpp	93.5% <60.0%> (-1.7%)	⬇️
decks/custom_init.cxx	100.0% <100.0%> (+67.9%)	⬆️
src/fields.h	98.3% <100.0%> (+1.0%)	⬆️

... and 1 file with indirect coverage changes

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[guangyechen]

@streeve 'ES_Field_Solver' should really be 'ES_Field_Solver_1D' in its current form. Let me fix that.

[guangyechen]

@streeve It looks like the CI is trying to test ES not in 1D (which it should only test in 1D). Do you know how the CI works and fix this? Thanks.

[guangyechen]

@streeve It looks like the custom_init test was broken. I just fixed it and pushed to your fixup_es branch.