based on contour integral, without apparent singularities.
Validated against old (Rémy/Céline) implementation: agrees,
except for strong deviations near q->0, where the new
implementation is smoother.
The analytical calculations are explained in App. C2 of the manual.
This resolves #1300.

Manual: polygon by line integral.

little improvements elsewhere.
Still todo: sort arguments in intro to form factor catalogue;
describe polygon ff computations.

start links to online tutorial;
start appendix on form factor computation