Better support for domain indices.

First, GridView::getRawElementData() now takes an optional fourth output
argument: domainIndices.
Second, GridFactory::createGridFromConnectivityArrays() takes an analogous
optional fourth argument that can be used to set the domain indices of
the elements of the newly created grid.
Corrected estimate of cluster distance in WeakFormAcaAssemblyHelper.

The previous estimate was too low (it used to be correct for spherical
clusters, but now we use bounding boxes).

Distance-dependent quadrature rules should now bring more time savings.
Merge branch 'master' of https://github.com/bempp/bempp

Conflicts:
	lib/grid/grid_factory.hpp
	python/bempp/grid/grid_factory.i