ShadowRegression error estimation is incredibly slow

[jdidion]

Currently we call out to the R package for the calculations. It is research code and probably not optimized for performance. I should be reimplemented in cython.

[jdidion]

Closing this. The Wang et al. Shadow Regression method should be considered deprecated and will be removed in a future version, in favor of a native python or Cython implementation of the Zhu et al. cubic spline method: https://bmcbioinformatics.biomedcentral.com/articles/10.1186/s12859-016-1052-3 (or a better method, if one exists by the time I get around to implementing it).