Added a 'mode' field to AcaOptions. It will let you choose between various
ways of building operators: the traditional one, one involving synthetic
operators and one involving in-place coalescence of blocks.

At present, this field is ignored. It will be taken into account in
subsequent commits.
Changed the signature of Space::discontinuousSpace() and documented it,
along with isDiscontinuous(). Implemented it in RaviartThomas0VectorSpace.
Integral operators can now be constructed in local ACA mode (formerly
called synthetic mode) by setting AcaOptions::mode to LOCAL_ASSEMBLY.
Local-ACA-mode Maxwell and modified Helmholtz operators now assemble
the internal operators using global ACA mode (as it is already the case
for Laplace).
Maxwell operators detect and handle unsupported ACA modes.
Minimised use of deprecated globalAssemblyBeforeCompression flag.
Use the strong admissibility condition whenever an operator is discretised with
two discontinuous spaces.