Use form.arguments() instead of extract_arguments(form). May be faster.
Minor fix in commented out lines in demo.
Add argument 'function' to allow project(u, V, function=f) to store
the result in a preallocated function f.
Merge branch 'martinal/topic-cleanup-ufl-algorithms'
Merge branch 'martinal/topic-add-function-argument-to-project'