Should provide indication to user of valid bounds for q as well as other parameters

[butler]

Currently several models are only valid within a certain q range or more often a qr range. Likewise polydispersity widths can often be broad enough that their tails go into invalid territory (currently sasmodels will automatically truncate the distribution at known limits). Maybe other types of constraints will show up as well. It would be good to have a way to flag the user during operation that they are outside those bound (besides indications in the documentation of course).