realPart(), imagPart() and conj() are now both in Bempp and Fiber namespaces.
Added preliminary code that lets operators indicate empty blocks.

(Useful for simulations of metals).
estimateRelativeScale() is now used in the implementation of scale().
Slightly reorganised output from assembleAcaOperator().
Removed unnecessary overloads of asDiscreteAcaBoundaryOperator() from
Complexified and TransposedDiscreteBoundaryOperator.
Added a new argument, interleave, to asDiscreteAcaBoundaryOperator().
Added copy constructor and assignment operator to _4dArray.
Added an unpermutedIndices() method to IndexPermutation.
Implemented interleaved merging of H-matrix blocks.

Currently a lot of debug messages are emitted -- will be purged in another
commit.
Added a small spherical mesh to tests/unit/meshes.
Commented out the line outputting relscale.
Removed a forgotten curly bracket.
Removed a debug message.
Extended docs of the Maxwell module.
Updated debug message in modified_aca.hpp to include scale.
Extended documentation of the relativeScale() function in assembly helpers.
Merge branch 'empty_blocks'
In unit tests of interleaved asDiscreteBoundaryOperator(), ensured that
meshes do not have common points.
Added a safety valve in relativeScale().
Cleaned up debug messages emitted from mergeHMatrices() and friends.