Revisions to Transverse Field Ising Hamiltonian builder

[ccoffrin]

@zmorrell, this is what I had in mind. I think most users of this feature will not need to the constant terms, so we can leave these out. Also gave the function a more descriptive name with the idea of supporting different Hamiltonians down the road. I also added some new tests targeting the constant_field capability more specifically, so that we won't regress on the bug you found.

[codecov]

Codecov Report

Merging #21 (b9a682f) into ham-builder (9ae8156) will increase coverage by 2.25%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           ham-builder      #21      +/-   ##
===============================================
+ Coverage        94.59%   96.84%   +2.25%     
===============================================
  Files                5        5              
  Lines              444      444              
===============================================
+ Hits               420      430      +10     
+ Misses              24       14      -10     

Impacted Files	Coverage Δ
src/simulate.jl	94.59% <100.00%> (+6.28%)	⬆️
src/simulate_de.jl	100.00% <100.00%> (ø)

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[zmorrell]

This looks good to me. I'll go ahead and merge it into the other pull request branch