Remove the GeometryType data member from (most) LocalFiniteElements

Most LocalFiniteElement implementations hold a GeometryType data member, which is returned when the 'type' method is called. This is wasteful: constructing a GeometryType is cheap, and therefore having the 'type' method return a freshly constructed GeometryType object at each call will certainly not be slower than the status quo. This patch therefore removes the data member.

What is more, for most LFE implementations, the return value of the 'type' method is even known at compile time. Therefore, this patch turns most 'type' methods into static constexpr ones.