sasview: init at 4.1.2

Sasview is a data fitting and analysis package for small angle
scattering data.  More information can be found at
https://www.sasview.org.

There is some unfortunate cruft to this commit.  SasView uses the
xhtml2pdf python package, which has a specific version of html5lib as
a dependency.  This module manually loads that version into sasview.
I haven't made xhtml2pdf available as its own package due to these
circumstances.