Restructure of Assembler

The current assembler structure is obscure and methods are scattered. An attempt to simplify this is to be made.

0.3:

• Move global assembly loop from ProblemStat to a dedicated new class (see !163 (closed))
• Add interface to use constraints in assembly

After 0.3:

• Find a better abstraction for GridFunctionOperator, Assembler, OperatorList
• (optional) Discuss compatibility with PDELabs