Move SeparateFields invocation to Create function

I'm planting a Symme Tree! Doesn't make sense for the class itself to do
the field separation when it doesn't also weave them back together at
the end. It can't, though, since Weave is a clip operation, so I've
moved the SeparateFields stuff up to Create_TurnsTile.
Collapse TurnsTile constructors
Move some constructor vars to initializer list
Make gcf and mod static member functions