Context directly accessible under bempp namespace
Making grids constant

Currently can't modify grids from python, so might as well be constant.
Later on, we may have to add both and check whether the C bit is constant or
not.
Grid that space refers to obtain from C++

This grid is constant.
operators in context are now a hierarchy of modules

This should make it possible to add docstrings to the modules, as needed. And
it should make it possible to look at the docstrings of the operator factories.