review and edit equations in documentation for oriented particles

[richardh]

Though the equations in the documentation for oriented particles, cylinders etc are all "correct" they are sometimes rather ambiguous or potentially confusing to users, and in many cases could benefit from changes to variable names.

(a) many have integral from 0 to pi of (F²(Q).sin(theta))dtheta but this turns into integral from 0 to 1 of (F²(Q))dU after a change of variable. Though the docs report what the code does, the integration range needs to be explicitly shown, and ideally the change of variable explained in all cases.

(b) theta is being used for both the angle of the particle axis to Q and as an orientation angle to the particle axis, likewise psi sometimes appears in the form factor as well as being an orientation angle. The form factor variables should be renamed.

[richardh]

Was not as bad as I thought, had fixed most of the issues previously.
Triaxial ellipsoid has been left as was, with a note added about phi angle changing meaning. Will close ticket.