Add `Interfaces` submodule

[goerz]

Extends the QuantumPropagators.Interfaces module

[codecov[bot]]

Codecov Report

Merging #40 (c39f4c3) into master (4b83f14) will decrease coverage by 4.60%. The diff coverage is 46.42%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   75.50%   70.90%   -4.60%     
==========================================
  Files           5        6       +1     
  Lines         298      354      +56     
==========================================
+ Hits          225      251      +26     
- Misses         73      103      +30     

Impacted Files	Coverage Δ
src/interfaces/generator.jl	46.42% <46.42%> (ø)