Add new area for dual use plugins

The detector description group is introducing dual use plugins. Dual use means: they should function within athena; they should function outside of athena.

The CMakefiles are derived from other CMakefiles in the athena repository and should build on current platforms, though this has mostly been tested outside of athena and fixes may be required.

Note: plugins are shared object files currently with names like libXXX*.[so,dylib].*, But these are not linked, they are loaded at runtime. Therefore they are not part of any development kit, header files are not present, and certain pieces of the CMakefile related to installation of development libraries are not present.

Note: we do not want to include bits of CMakefile from AtlasCMake or AtlasLCG since this will break distribution outside of athena.

Suggestions welcome!