Performance improvements in generation of FESpaces

[fverdugo]

Some improvements in the performance of FESpaces are needed, in particular:

• Improve efficiency of low level routines that generate dofs, do the interpolation etc (i.e. introduce function barriers and avoid fine grained allocations). Fixed since PR Enhancements in fespaces #54
• Improve efficiency of the interpolation on Dirichlet boundary. (i.e., only loop over boundary cells, some times several tags can have the same function associated to them)