Misc docstring fixes in core_types
Return None for op2.Halo if sends and receives are empty
Use make_dat to create Dat on a FunctionSpace
Only flatten Args on VectorFunctionSpaces
Sphinx config: use docstrings of class and __init__ method
Docstring fixes for solving
Merge pull request #50 from firedrakeproject/misc

* Only flatten Args on VectorFunctionSpaces
* Use make_dat to create Dat on a FunctionSpace
* Return None for op2.Halo if sends and receives are empty
* Docstring fixes for core_types and solving