Documentation: Sphinx doesn't properly format many dolfin.cpp functions

[Marie Rognes]

Sphinx is having a hard time autodoc-ing many of the dolfin.cpp classes/functions. This shows up as O(10^3) warnings when running Sphinx for the Python documentation, _and_ as not properly formatted function arguments. See dolfin.cpp.NewtonSolver.solve as an example (compare C++ and Python).

The reason for this is that Sphinx tries to extract the types of the arguments and the return arguments from the code, and does not understand how to relate to the _cpp module. (I think.)