Add Interfaces submodule

[goerz]

Add routines to test whether user-defined objects match the expected interface for generators, operators, states, amplitudes, and controls

[codecov[bot]]

Codecov Report

Merging #50 (01046a4) into master (8555759) will increase coverage by 1.41%. The diff coverage is 81.18%.

:exclamation: Current head 01046a4 differs from pull request most recent head 96815f0. Consider uploading reports for the commit 96815f0 to get more accurate results

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   85.38%   86.80%   +1.41%     
==========================================
  Files          16       21       +5     
  Lines        1197     1500     +303     
==========================================
+ Hits         1022     1302     +280     
- Misses        175      198      +23     

Impacted Files	Coverage Δ
src/amplitudes.jl	83.92% <71.42%> (+55.92%)	:arrow_up:
src/interfaces/control.jl	74.19% <74.19%> (ø)
src/interfaces/operator.jl	76.71% <76.71%> (ø)
src/interfaces/generator.jl	78.00% <78.00%> (ø)
src/interfaces/amplitude.jl	86.04% <86.04%> (ø)
src/interfaces/state.jl	86.86% <86.86%> (ø)

... and 2 files with indirect coverage changes