Draft: Add P1+B simplex bubble functions

Summary

This MR adds a local finite-element representing linear Lagrange functions enriched with a single bubble-function b_T that is defined as the product of the linear basis functions.

The basis functions are implemented for simplex elements only in dimension 1,2, and 3.

Requirements

• Needs core/dune-localfunctions!257 (merged) to be merged
• Is based on !436 (merged)