Reintegrate feature branch cdo-magics-build for improving the building...

Reintegrate feature branch cdo-magics-build for improving the building environment for Magplot operators

Current status:
The plotting operators are only available, then CDO is condfigure with magics
AND libxml2. Bot are now available as regular configure options - manual
setting of CPPFLAGS or LDFLAGS is not necessary anymore.
All CPPFLAGS for MAGICS and XML have been removed from the related source code files. Only two files are envolved:

* src/modules.c: adding the corresponding function pointers and operators
* src/Makefile.am: adding the related source code files to the list of source files of CDO

TODO:

* add documentation on how to use plotting
* make the documenation conditionally available. This should already work for the build-in help, but not for the pdf docu