[ENH] Add iterative masking EMD

[ryanhammonds]

Addresses #26 by adding itEMD, which helps prevent mode mixing.

[codecov-commenter]

Codecov Report

Merging #27 (b942919) into master (d8d348a) will decrease coverage by 0.05%.
The diff coverage is 80.95%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
- Coverage   96.97%   96.92%   -0.06%     
==========================================
  Files          21       21              
  Lines        1356     1366      +10     
==========================================
+ Hits         1315     1324       +9     
- Misses         41       42       +1     

Impacted Files	Coverage Δ
ndspflow/optimize/group.py	91.30% <75.00%> (+0.82%)	⬆️
ndspflow/optimize/refit.py	93.60% <77.77%> (-0.62%)	⬇️
ndspflow/optimize/emd.py	100.00% <100.00%> (ø)

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