Plot3Plot: allow callers to explicitly specify the maths type to use
Move maths type autodetection into Fractal where it belongs.
Overhaul maths traits so that the master list lives in precisely one place.

N.B. The meat of this commit is in Fractal.h and Fractal.cpp; other changes are just applying it.
Fix b2benchmark build deps
Tweak benchmarks.
Add float maths type
Allow float and double to be configured out, as they don't seem to speed things up on amd64