Feature/md explicit assembler

Steps to do:

• cleanup of md-cc-localassembler
• add documentation
• introduce explicit local assembler
• implement tpfa test
• try to reuse cc-explicit-local assembler (assembly routine is essentially the same) -> discarded
• implement box-explicit multidomain local assembler
• implement test for box
• use new namings/folders according to !1284 (merged)
• add reference files to tests

Fixes #521 (closed)