Add a finite volume method for ScalarEq

Proof of principle.

Introduced a (suboptimal) extra FVElement type and added another type parameter to Mesh. Added a ref test for the advection eq with periodic sine wave.

This PR also did some refactoring of the ScalarEq project:

• remove all occurrences of the old broadcast methods (now using broadcast_*_2! or newer)
• introduced some abstractions for Mesh to compactify time step computation

So far only 1d is implemented!

added 1d feature parameters refactor labels

mentioned in commit 5a594e69

merged

mentioned in commit ba310be2

mentioned in merge request !91 (merged)

mentioned in commit fe3f041c