Initial implementation of function expansion by interpolation.
Removed code that printed the number of integrals handled by each integrator.
GridFunction now doesn't convert projections into coefficients until it
becomes necessary.

The signatures of setProjections() and projections() have changed
(the old versions are still available, but deprecated.)
Removed a line that printed chunkSize.
Fixed a bug in PiecewiseConstantScalarSpace::getGlobalDofBoundingBoxes():
the gdof reference positions are now correctly set to element centres
rather than their first vertices.
PiecewiseConstantScalarSpace::getGlobalDofNormals() now works on grid segments.
Improved documentation of some member functions of Space.
Added to Space functions explicitly responsible for interpolation.

(We now distinguish between getGlobalDofPositions(), used in clustering,
and getGlobalDofInterpolationPoints(), used in interpolation. Note that
a space might not be interpolatory, in which case
getGlobalDofInterpolationPoints() will throw an exception, whereas
getGlobalDofPositions() can still work, perhaps returning the element centre
for all the basis functions located on an element.

The mechanism is sufficiently general to allow interpolation of vector
(e.g. surface-tangential) functions, although it is not yet implemented
for the particular case of Raviart-Thomas spaces.
Fixed an incorrect assert.
Merge branch 'release_2.0' of https://github.com/bempp/bempp into release_2.0