idwtn should allow coefficients to be set as None

[grlee77]

The current behavior in idwtn does not match the description in the docstring when any of the values in the coeffs dictionary is None. This PR updates the behavior to match the docstring. This also makes idwtn treat None coefficients in the same was as idwt2.

Previously idwtn raised an error if any coefficients were None.

technically that is a change in behavior, but as it is a bug fix I don't think we should need a deprecation cycle?

closes gh-290

[codecov-io]

Codecov Report

Merging #291 into master will decrease coverage by -0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #291      +/-   ##
==========================================
- Coverage   86.36%   86.26%   -0.11%     
==========================================
  Files          21       21              
  Lines        3088     3086       -2     
  Branches      532      532              
==========================================
- Hits         2667     2662       -5     
- Misses        368      371       +3     
  Partials       53       53

Impacted Files	Coverage Δ
pywt/_multidim.py	97.77% <100%> (ø)	✅
pywt/_extensions/_swt.pyx	75.96% <0%> (-3.29%)	❌

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[rgommers]

technically that is a change in behavior, but as it is a bug fix I don't think we should need a deprecation cycle?

I agree

[rgommers]

LGTM for idwtn, however as far as I can tell the idwt2 behavior is not documented or tested. Maybe good to add that?

[grlee77]

Added an idwt2 test and updated the docstrings to describe the expected behavior.

[rgommers]

Merged, thanks @grlee77