Add `check_propagator` routine

[codecov[bot]]

Codecov Report

Merging #53 (f915fd8) into master (914f469) will increase coverage by 2.79%. The diff coverage is 83.73%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   87.20%   89.99%   +2.79%     
==========================================
  Files          21       22       +1     
  Lines        1508     1629     +121     
==========================================
+ Hits         1315     1466     +151     
+ Misses        193      163      -30     

Impacted Files	Coverage Δ
src/interfaces/generator.jl	80.00% <ø> (ø)
src/interfaces/operator.jl	76.71% <ø> (ø)
src/propagator.jl	75.00% <ø> (+13.88%)	:arrow_up:
src/pwc_utils.jl	97.67% <ø> (ø)
src/interfaces/propagator.jl	83.19% <83.19%> (ø)
src/interfaces/state.jl	88.88% <100.00%> (ø)
src/newton_propagator.jl	97.05% <100.00%> (+11.76%)	:arrow_up:

... and 3 files with indirect coverage changes