Greatly extend and improve the test file for adaptors (#67)

- Add concept checks for the alterable, extendable, erasable
and clearable adaptors.
- Add test cases for modifying the underlying graphs through adaptors
whenever it is possible.
- Check the conversions between Node, Arc and Edge types.
- Add more test cases for the adaptor-specific functions and maps:
enable(), disable(), status(), forward(), backward(), CombinedArcMap,
CombinedNodeMap, ResidualCapacity etc.
- Use checkGraphIncEdgeArcLists() to simplify the test cases for
undirected graphs.
- Add test cases that use static graph structure (GridGraph) with
several adaptors combined.
- Add comments for the test cases.

 INCLUDE(FindPackageHandleStandardArgs)

 FIND_PROGRAM(GHOSTSCRIPT_EXECUTABLE

   NAMES gs gswin32c

   PATHS "$ENV{ProgramFiles}/gs"

   PATH_SUFFIXES gs8.61/bin gs8.62/bin

   DOC "Ghostscript: PostScript and PDF language interpreter and previewer."

 )

 FIND_PACKAGE_HANDLE_STANDARD_ARGS(Ghostscript DEFAULT_MSG GHOSTSCRIPT_EXECUTABLE)