user defined distribution functions

[pkienzle]

Allow distribution functions to be defined much like form factors and structure factors by placing a distribution definition into the plugins directory. The model loader would recognize it as a distribution and add it to the list of distributions.

The UI will need to be extended to allow more distribution shape parameters. Currently it is limited to a single shape parameter (width), which is tied to the parameter type (orientation parameters use absolute width, volume parameters use relative width).

Existing distributions should be moved into the sasmodels/models directory.

[pkienzle]

Partly addressed by

​https://github.com/SasView/sasmodels/pull/67/

This uses the ~/.sasview/weights directory for weights plugins, but does not provide any UI support for updating the list of weights, nor does it allow additional parameters on the distribution.