warn user about invalid model approximations

[pkienzle]

There are several areas of concern with model correctness:

• Nevot-Croce is valid only over a limited domain
• dz/dA may be too large for the Q range
• point density may be too low for the resolution to be computed accurately

We could automatically check all of these by computing a fine sliced model over a dense Q range and compare this with the users model. This could be done when the model is first loaded and after the fit is complete.