Add `check` argument to `propagate`

[goerz]

This allows to check the tlist, generator, and state before doing the propagation.

[codecov[bot]]

Codecov Report

Merging #51 (f6e61a1) into master (cfd9781) will increase coverage by 0.25%. The diff coverage is 62.50%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage   86.87%   87.13%   +0.25%     
==========================================
  Files          21       21              
  Lines        1501     1508       +7     
==========================================
+ Hits         1304     1314      +10     
+ Misses        197      194       -3     

Impacted Files	Coverage Δ
src/propagate.jl	87.27% <62.50%> (-4.40%)	:arrow_down:

... and 4 files with indirect coverage changes