Various bug fixes and code improvements in adaptors.h (#67)

- Fix UndirectorBase::nodeNum().
- Fix UndirectorBase::findEdge().
- Fix OrienterBase::addArc().
- Fix OrienterBase::findArc().
- Improve SplitNodesBase::findArc().
- Add missing notifier() function in UndirectorBase.
- Add missing typedefs for maps (conform to the ReferenceMap concept).
- Add some useful typedefs for graph adaptors.

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)