Allow user to customize NXcanSAS loading options

[krzywon]

As noted in SasView/sasview#1639, allowing the user to define the data paths for nexus data would create a more flexible reader without having to redefine NXcanSAS. This will likely require a configuration in this package that is then read in by sasview in some meaningful way. This could then be integrated into the preferences window started in SasView/sasview#2167.