Check all model documentation for standardization

[butler]

In particular ensure that sections are in standard order, all equation images are removed and that reference and reviewed by sections are included.

[butler]

Not critical for 4.0 - documentation is OK if variable. Will move this to 4.1 so we can release 4.0 now.

[butler]

In adc753dbe4450955415a1427496ad3341a93896e/sasmodels:

bicelle model documentation standardized, ref to original derivation and
equations added to rst documentation. re #696 and #646

[butler]

In 755ecc2c8863bbe3bdb29424e14f1f9a805f7e6e/sasmodels:

Update documentation to meet standards. addresses #646

[butler]

In 5810f00b5c1f61f1386ab0be011468bf48fbf8c0/sasmodels:

updating documentation to meet standards. Addresses #646

[butler]

In 98ce141a6e091b02f8139ab3b235c16dafbce663/sasmodels:

correct normalization problem in stacked_disk fixes #789. Also heavy
cleanup and correction of equations to match code as well as
standardization of the model documentation addressing #646.

[butler]

In 07300ea35d16b40f790fffc4ded20189ec5267a4/sasmodels:

update, correct and standardize documentation for fractal_core_shell as
well as small tweak to stacked_disk documentation. Addresses #646

[butler]

In 5d23de23656b990e141f01721eb85cd0afaa4dc2/sasmodels:

Fold caveats from original IGOR documentation back into sasmodels
version as requested on sasmodels pull request 27 addressing ticket 843.
Also reformated refs and authorship section as per ticket #646

[butler]

In d439007cd436f6eef212f3d956711e6f3d29f2fb/sasmodels:

Corrected errors in star_polymer documentation identified by MN student
checked carefully against code, benoit paper and Richter paper cited in
ticket. Also took the opportuinty to generally udated and standardize
the documentation as per ongoing process addresses #646. fixes #962

[smk78]

In addressing #883, #1187, and #1240 I have been through every single P(q) and S(q) model file and:

• corrected typos where spotted
• changed wording/layout in places (for consistency, standardisation, etc)
• added some additional references
• ensured all files have Authorship & Verification sections (even if some fields are empty!)
• added links to the source code on GitHub
• checked all models have equations (except where noted in the tickets above)
• removed a legacy png formula

I now consider that the look and content of the model documentation is in pretty good shape and that this ticket can now be closed. Specific issues should now be raised in specific tickets.