Remove boost type erasure

Part of !527 (closed):

Drop type erasure for Grid and InterpolatedBFieldMap. The templating now bubbles through to the stepper. Setup becomes little more complicated, but we're not going through TE at all anymore, which should be a performance benefit. Effectively removes any signature requirements on field mappers and the grid, even though they were not enforced even before, since that only happened when using the concepts with TE.

Depends on !531 (merged) to be merged first.