document reasonable q and parameter limits

[pkienzle]

For each model, list the range of q, r, etc. for which the model is accurate to within epsilon.

Use the same epsilon across the models (e.g., 5e-5 for single precision, 5e-10 for double precision).

Note that the special function libraries in sasmodels are tuned for single and double precision only, so comparing against -quad! may give misleading results. Use the explore/precision.py program to examine the valid range for the underlying sasmodels special function library.