Building Python modules through Pybind11 is a recurring task in our
Python bindings. To reduce maintainance in the build system, we introduce
a CMake function "dune_add_pybind11_module", which is semantically inspired
by the "dune_add_test" function.

if DUNE_PY_DIR set use that
else if in virtual env put it in VIRTUALENV_PREFIX/.cache/dune-py
else in HOME/.cache/dune-py

This module simply imports everything from cPickle or pickle, depending on
the Python version.

included for this function

notebook execution).
Also improve error handling in ConfigurationTest

For Python 2.7, __package__ seems to be None, while __name__ is the fully
qualified module name (e.g., dune.common.module). This is also sufficient to
distinguish whether module.py was loaded from the dune package or not.

Previously, there might have been several matches of identifier in
config.h, though only one of them was a definition. This caused a
false positive.

preprocessor tests

of a HAVE_FOO in config.h can be tested for. A possible usage is shown in
spgrid/_grid.py
Other checks can be added in a similar manner

Regain the full potential of the old create mechanism:
import dune.create as create
grid = create.grid("OneD", ...)

dune.create automatically gathers all creatable objects from all DUNE
modules declaring their 'python-modules' in dune.module. Such a Python
subpackage has to export a module _create.py containing a dictionary
called 'registry'. This dictionary maps the type of creatable objects to
another dictionary, which in turn maps a string to the actual creation
function. In case of dune.grid, this is

registry = {}
registry["grid"] = {
        "Alberta": albertaGrid,
        # ...
    }