add dx methods

[oscardssmith]

Before

const bwd = Diffractor.PrimeDerivativeBack
bwd(bwd(x->5))

failed.

[Keno]

Needs tests

[codecov-commenter]

Codecov Report

Merging #77 (3dbc84e) into main (be3b62d) will decrease coverage by 0.04%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main      #77      +/-   ##
==========================================
- Coverage   52.67%   52.62%   -0.05%     
==========================================
  Files          21       21              
  Lines        2170     2172       +2     
==========================================
  Hits         1143     1143              
- Misses       1027     1029       +2     

Impacted Files	Coverage Δ
src/interface.jl	70.90% <50.00%> (-0.79%)	⬇️
src/stage1/forward.jl	76.87% <0.00%> (-1.37%)	⬇️
src/tangent.jl	34.04% <0.00%> (+1.06%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update be3b62d...3dbc84e. Read the comment docs.

[oscardssmith]

this is ready to merge.