Add -sesans option to sascomp simulator

[wpotrzebowski]

@wpotrzebowski to take a look at it.

[butlerpd]

does not affect GUI. May not affect most scripting folks .. mostly part of @pkienzle testing infrastructure so should be fairly safe? Wojciech will take a quick look but will otherwise merge by next call at latest.

[pkienzle]

Options for nxsunit:

• Move nxsunit from sasview to sasmodels,
• Move the data loader into its own package with nxsunit,
• Move nxsunit into its own package, maybe with other python nexus support routines,
• Hard code angle and inverse distance units and transforms in sasmodels
• Only import nxsunit conversion if the data units do not match the target units

I'll implement the last option. The simulation code can set up the vectors in the target units directly.

If you are using sasmodels to fit measured data you presumably have the data loader on the path so the unit conversion code will be available. So long as nxsunit stays with the data loader the lazy loading will work.

Splitting nxsunit into a separate package is worthwhile. Any generic NeXus file reader will need something like this to handle the idiosyncratic units that can appear. The code was originally written for the nexus api wrapper here. Another variant is in the NCNR reductus package here. Maybe combine it with other NeXus data read/write utilities.