"invalid q range: qmin must be smaller than qmax" on a number of models

[ajj]

From Paul Butler's testing on 28th August 2016:

Spherical_sld, lamellar_stack_paracrystal, stacked_disk and square_well all generate a very strange error before appearing to work: "invalid q range: qmin must be smaller than qmax," but then appear to work. Note that in these tests the condition (qmin is smaller than qmax) is always true as it is just the default qmin and qmax in the fitpage.

[pkienzle]

Happens on all models if Value matches Min or Max. For example, set background to 0.

[Paul Kienzle <pkienzle@…>]

In ee4b3cbc1850f87671526c54335834959ffdfe84:

fix 'invalid Q range' when parameter value is min or max. Fixes #636