FunctionSpace overhaul

[cmhamel]

• Rename to L2Field
• Make Connectivity a seperate type
• remove L2ElementField
• In meshes make element conns a Dict like everything else. Last bit of NamedTuples left I think
• Re-add dirichlet bcs to fspace since we'll need to update these if we raise order
• Need to add block level element id maps for updating polynomial order
• Add methods to extract linear edges
• Add methods to extract linear facets
• H1 space
• L2 space
• Hdiv space
• Hcurl space