Improved the (debugging) routine for dumping dense blocks.
Added a function forcing a certain cluster to be approximated first.

(Useful for debugging ACA.)
Added an assertion to local_dof_lists_cache.cpp.
Moved #include <approx.h> from ahmed_aux.hpp to aca_global_assembler.cpp.

In this way a change to ACA only forces recompilation of a single object
file.
Removed a leftover comment from maxwell_photonic_jet.py.
In dumpDenseBlocks(), added requirement of admissibility.
Added experimental code for possibly faster evaluation of dense blocks.
To be profiled.
Added optional diagnostic output from the constructor of
AcaApproximateLuInverse.
Removed override of applyImpl() in AcaApproximateLuInverse,
which was identical to that in base class.
Added several patches of AHMED code.
Added a modified ACA procedure.

It contains new heuristics intended to deal with the presence of
block structure in approximated matrices.
Included one more AHMED patch.
Corrected whitespace and paths in patches of AHMED.
AHMED patch no longer zeroes pu.
Documented the useAhmedAca option.
Merge branch 'new_aca_algo'