document limits on parameter ranges for Hayter-Penfold structure factor

[pkienzle]

Setting the charge to 204 in the following parameter set breaks the Hayter_MSA model:

sasview_parameter_values:scale,False,1,0,inf
:background,False,0.001,-inf,inf
:sld,False,3.5,-inf,inf
:sld_solvent,False,6.3,-inf,inf
:radius,False,300,0,inf
:volfraction,False,0.2,0,0.74
:charge,False,204,0,250
:temperature,False,293,0,450
:concentration_salt,False,0.02,0,inf
:dielectconst,False,78,-inf,inf

The docs don't say anything about it.

[pkienzle]

Note that the default parameter range is limited to 200, so this isn't a problem in practice.

[richardh]

This ticket is perhaps really part of #859. 

The Hayter-Penfold rmsa S(Q) has a lot of "instabilities" which may be due to the physical limitations and/or numerical approximations used in generating it. Paul Butler and Richard had some discussions by email with Paul Fitzgerald in Sydney in Feb & March 2017 regarding issues he had seen. Jeff Penfold's view is that this S(Q) should not be pushed too far. For the record, an editied version of the email string is about to be included on #859

[smk78]

The model docs for hayter_msa now include a specific note about the charge limits, and I have added comments to the source code that reference this ticket and ticket #859. The purpose of this ticket has therefore been met and I am closing it.