Convenience methods for all at once assembly

• Rename assemble(Matrix|Vector)() to assemble(Matrix|Vector)Values().
• Add convenience methods assembleMatrix() and assembleVector() for all-at-once assembly including resizing, building the pattern, initializing the matrix, and assembling the values.

This helps to avoid boiler plate code in the common simple cases (see modified examples) but leaves full control in the complicated ones (see 2nd assembly code path of Stokes example).

The following are no longer part of this MR and have been implemented separately before:

• Import missing operator= in ISTLVectorBackend.
• Extract patter.resize() from assembleMatrixPattern(). For consistency and composeability the user should call this manually.