Add docstrings for PDFEnvelope classes.
Clean up pickling for ScatteringFactorTable.
Clean up the _registerDoubleAttribute wrap.

Move python code to python file.
Remove unnecessary imports from wrapper functions.

Objects should be imported from Python modules and not from srreal_ext.
Injection of convenience funcitons would happen upon imports.
Move PDFEnvelope classes to pdfenvelope module.
Move PDFBaseline classes to pdfbaseline module.
Move PeakWidthModel classes to peakwidthmodel module.
Add explicit wrappers for GaussianProfile and CroppedGaussianProfile.
Move PeakProfile classes to peakwidthmodel module.

Also support pickling for PeakProfile classes.