Added the module Grid to documentation.
Various documentation enhancements.

Mainly:
- added \relates tags to standalone functions
- put all Concrete* classes into grid_internal group (which does not
  figure in the Modules list, on purpose)
- simplified docs of some classes from linalg
Corrected include paths in the generated C++ docs.
Added a search field to the generated Doxygen documentation.
Added a missing include to bempp.swg.
Updated README and VERSION.