[python][forms] Various bugfixes and improvements to python bindings and examples
Backport various improvements from my lecture examples:
- [bugfix] Enclose sum in
(a+b)
. - Use namespace for some forms operators to avoid ambiguity.
- Add python bindings for
bindToCoefficients()
. - Allow to use python assembler in non-verbose mode.
- Add two nonlinear python examples (semilinear and minimal surface equation).