Cleanup/periodic bc

What this MR does / why does DuMux need it:

Renames the perodic map and uses this map in the assembler.

Notes for the reviewer

TODO: insert text here

Before you request a review from someone, make sure to revise the following points:

• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.