Fix warnings

[streeve]

Mostly unused variables and Kokkos parallel labels

[streeve]

@guangyechen this should just be cosmetic changes to remove (almost) all the warnings

[codecov]

Codecov Report

Merging #46 (9d669bd) into master (ca04ad6) will not change coverage.
The diff coverage is 95.2%.

@@          Coverage Diff           @@
##           master     #46   +/-   ##
======================================
  Coverage    78.9%   78.9%           
======================================
  Files          12      12           
  Lines         801     801           
======================================
  Hits          632     632           
  Misses        169     169           

Impacted Files	Coverage Δ
decks/custom_init.cxx	32.0% <ø> (ø)
src/helpers.h	51.4% <50.0%> (ø)
example/example.cpp	95.1% <100.0%> (+0.1%)	⬆️
src/accumulator.cpp	100.0% <100.0%> (ø)
src/fields.h	97.2% <100.0%> (-0.1%)	⬇️
src/input/deck.h	78.7% <100.0%> (ø)
src/interpolator.cpp	100.0% <100.0%> (ø)
src/move_p.h	72.2% <100.0%> (ø)

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