Removed backup files accidentally committed some time ago.
Replaced last instances of old-style uses of AcaGlobalAssembler::assembleDetachedWeakForm().
Incompatible change: removed all classes denoting particular Laplace,
Helmholtz and modified Helmholtz operators and made factory
functions like laplace3dSingleLayerBoundaryOperator() create instances of
GeneralElementarySingularIntegralOperator instead. Factory functions
creating Helmholtz operators internally use those associated with
modified Helmholtz operators.

In addition, implemented synthetic assembly for all boundary operators
associated with the Helmholtz and modified Helmholtz equation and improved
documentation of factory functions.
Extended documentation of two Helmholtz operators.
Corrected an include guard.
Formatting correction.
Brought a comment up to date.
Implemented synthetic assembly of the Maxwell single-layer operator.
Removed some leftover comments.
Various documentation corrections.
Added the module Grid to documentation.