[test.spaces] use conform variant of alugrid
[test.operators_projection] added

This commit introduces detailed tests of the projection operators. A base
class is provided in test/operators_projections.hh that checks the
projection of a linear function for correctness. There exists another
header for each projection operator type (checking nothing additionally
atm) and a test source for each projection operator/space type/space
backend type combination.
[test.operators_oswaldinterpolation] added

This cleans up the old test. Note though that this is not yet a test for
mathematical correctness.
[test.operators_darcy] added

This cleans up the old test.
[test.operators_products] removed obsolete header
[operators.projections] fixed L2Projection
[operators.projections] added dg.pdelab space
[operators.projections] added note