Section on twinning needs work

[luc-j-bourhis]

[Richard's point 22] Perhaps the section on twinning, especially the description of
different kind of twins could be reduced, and we just refer readers to the
various Herbst-Irmer & Sheldrick papers. We could just explain the
distinction in treatment of twins where the reciprocal lattices are
(almost) exactly superimposed and non-merohedral twins (although I prefer
the terminology "multiple lattices" here now). Speaking of the latter,
hasn't Oleg implemented support for hklf5 refinement in smtbx/olex2.refine?
Perhaps Oleg should add a few words here regarding his implementation.

[luc-j-bourhis]

I would appreciate if you and Oleg sorts this out as my contribution to this area amounts to exactly 0.