Revise documentation for Guinier-Porod model

[smk78]

@smk78 & @butlerpd note that there are some confusing statements and missing references in the Guinier-Porod model documentation.

The current documentation replicates that written by Hammouda in 2010 (see Ref 1 here) for Kline's IGOR-based fitting application from NIST.

Both sets of documentation state:

For 3D globular objects (e.g. spheres), s = 0 recovering the standard Guinier formula. For 2D symmetry (e.g. rods) s = 1, and for 1D symmetry (e.g. lamellae or platelets) s = 2.

The assignment of s values to shapes matches the Hammouda reference (see bottom of page 717 here). What seems wrong is the assignment of the symmetry to the shapes; ie, should not rods be 1D and lamellae/platelets be 2D? For that matter, what exactly is meant by 'symmetry' in this context anyway? It might be less confusing to just refer to the shapes and s values and omit all mention of symmetry?

The Sasmodels documentation further states:

overcomes some of the deficiencies of the (Beaucage) unified_power_Rg model

and

(see the Glatter reference below)

Neither reference for Beaucage or Glatter are provided, although Beaucage's Unified model in Sasmodels is linked.

The Glatter reference is provided in Kline's documentation as:

Glatter, O.; Kratky, O., “Small-Angle X-Ray Scattering”, Academic Press (1982). Chapter 4 on Data Treatment, pages 155-156.