Refine timing system

[jorgensd]

Currently we time the whole assembly, including creating the matrix (and sparistypattern).
This is not optimal, as the sparsity pattern might be alot smaller if assembling over set of sub entities (for instance exterior facets).

If we want to have accurate comparisons, we need to change the signature of the assembly function to have the matrix as input (dolfin-x already has this option).