Feature/cleanup nedelec basis

Cleanup NedelecFiniteElementMap

• Use LFEVariant instead of virtual interface for mixed grids. This allows to have value-semantics.
• Use a single container to store all required variants.
• For a single GeometryType grid only store the needed variants.
• For a mixed grid store all variants using LFEVariant.

Benefits:

• Less complicated storage
• No GeometryType-dependent look-up required in find

This is a follow up to !280 (merged)