WIP: [assembler] Add parallel assembly with tbb and coloring

• Look at other discretization schemes (or at least disable feature for non-tpfa, non-box)
• Add guards in case TBB is not available.
• Guard for gridviews which are not thread-safe.
• Check caching (due to coloring this should also work with caching)
• Option to set number of available threads at runtime

Check efficiency of coloring scheme (efficiency doesn't matter much for instationary simulations).

The constraint behind the coloring should be: Two elements that modify the same entries of the matrix or the cache or any other global object shouldn't have the same color. In the graph coloring sense: Two elements (two nodes) that do modify the same entries are connected by an edge.