update SVD shapes in PIRLS loop

[codecov[bot]]

Codecov Report

Merging #161 into master will decrease coverage by 0.34%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
- Coverage   91.61%   91.26%   -0.35%     
==========================================
  Files          16       16              
  Lines        2146     2153       +7     
==========================================
- Hits         1966     1965       -1     
- Misses        180      188       +8

Impacted Files	Coverage Δ
pygam/pygam.py	89.65% <100%> (+0.09%)	:arrow_up:
pygam/links.py	84.61% <0%> (-15.39%)	:arrow_down:

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[dswah]

OK, in this PR i've primarily changed 3 things:

• [x] allow SVD shapes to change during PIRLS iterations due to changing masks for good data. this solves https://github.com/dswah/pyGAM/issues/150
• [x] use a different initialization point for the model coefficients. we now begin from a constant model
• [x] GammaGAM, and InvGaussGAM use non-canonical link functions by default. they are set to log link for now, since it is difficult to enforce positive mean otherwise