Improve and enhance HRF basis model

[mwaskom]

• Add new glm.GammaBasis class for informed basis set model
• Add dispersion derivative to glm.GammaBasis (but not used in lyman workflows)
• Orthogonalize predicted responses for the derivative regressors w/r/t main regressor
• Simplify output of glm.GammaHRF
• Change parameter name of HRFModel.transform to avoid collusion with built-in

[codecov-io]

Codecov Report

Merging #192 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #192   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          10     10           
  Lines        2304   2320   +16     
=====================================
+ Hits         2304   2320   +16

Impacted Files	Coverage Δ
lyman/workflows/model.py	100% <100%> (ø)	:arrow_up:
lyman/glm.py	100% <100%> (ø)	:arrow_up:

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