bcc_paracrystal and fcc_paracrystal scaling, docs for all paracrystal

[richardh]

Following some discussions with Prof. Gustavo González Gaitano, University of Navarra, Spain and some test calculations.

Given that the docs are already modified for the new "jitter" integrals (in progress??) which branch(es) should I use to fix the below please?

It now seems clear that:

1) The docs for all three paracrystal models talk about "nearest neighbour distances", but these should be lattice spacings, as previously noted in #805

2) the Vlattice corrections mentioned in the docs seem wrong for BCC and FCC, and need to be changed there and in the code in order for fits to have the correct scale parameters.

I think that latticescale should be as for SC, but with the multiplicities in for FCC and BCC thus -

Instead of:
FCC const double s1 = dnn*sqrt(2.0);

const double latticescale = 4.0*sphere_volume(radius/s1);

BCC const double s1 = dnn/sqrt(0.75);

const double latticescale = 2.0*sphere_volume(radius/s1);

SC NB: 4/3 pi r^{3 / dnn}3 = 4/3 pi(r/dnn)³

const double latticeScale = sphere_volume(radius/dnn);
answer *= sphere_form(q, radius, sphere_sld, solvent_sld)*latticeScale;

we need for FCC and BCC:

FCC

const double latticescale = 4.0*sphere_volume(radius/dnn);

BCC

const double latticescale = 2.0*sphere_volume(radius/dnn);

and in some other places for IQxy

[butler]

These are model docs right? In that case you can do it directly in master for now (those are in sasmodels not SasView)