pure python valid region does not behave as shown in docs

[butlerpd]

If there are regions of parameter space which are not valid, a model function can exclude them from fitting according to the docs. However the C models do it differently and require a valid = (Boolean expression) while the python model is told to return np.nan.

Recent attempts at using this second version shows it does not actually work for fitting. Upon investigation @pkienzle believes that the following code placed at line 225 of kernelpy.py] might fix the problem

total = form()
if np.isnan(total).any():
    total = np.zeros(nq, 'd')

That said, a preferred solution would be to make the two interfaces the same. That however would require parsing a C style Boolean expression.

[pkienzle]

Should avoid computing the form volume and form radius since they are also likely to be invalid. The code becomes:

if call_details.num_active == 0:
    weight_norm = 1.0
    total = form()
    if np.isnan(total).any():
        total = np.zeros(nq, 'd')
        weighted_shell = weighted_form = weighted_radius = 0
    else:
        weighted_shell, weighted_form = form_volume()
        weighted_radius = form_radius()