[GRASS-dev] gpde library updates

[Sören Gebbert]

Hi,
i just committed some gpde library updates.
Including:
* different mean calculation algorithms (arithmetic, geometric, harmonic and quadratic mean)
* upwidning stabilization
* better solute transport implementation
* fixed gradient field computation
* documentation update
* and much more

I have moved the gpde global header files
from grass6/include to grass6/lib/gpde. The header files are copied
before compilation to $(GISBASE)/include/grass/. (Thanks for the explanation Glynn)

All global definitions and typedefs are now available in the
doxygen documentation.

Best regards
Soeren