fixed some bugs

This allows them to be loaded without loading the corresponding Python
mdoule, avoiding circular dependencies.

to reduce runtime (checking for the module is quite expensive).

x[:] now returns the underlying numpy buffer and x[1:3] the slice
not that x is itself not a numpy array, so for transpose one needs
x[:].transpose() to get the transposed view of the field vector.

The same works for entity(x)[:] but also now
localpoints = numpy.array([lx1,lx2,lx3,lx4],[ly1,ly2,ly3,ly4]]
coords = entity(numpy.array(localpoints)
coords[0] = numpy.array([gx1,gx2,gx3,gx4])
with (gx1,gy)=entity.geometry.position((lx1,ly1))
coords[:] = numpy.array([[gx1,gx2,gx3,gx4],[gy1,gy2,gy3,gy4]])
etc.

Note that the global coordinates are computed when coords[] is called so only
calling coords.local[] does not lead to overhead

- The method 'domain' is at the moment removed from the entity.geometry
  (I might want to add it at a later point possibly on the entity)
- quadrature/referenceelements are not exported anymore by the hierarchical
  grid module
- new method for obtaining the reference element and quadrature rule for a
  given geometry type are added to dune.geometry

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.