Snagging of the new classes

[davidorme]

There are some minor things that need fixing up:

• Remove PModelABC.gpp_conversion_factor and the related PModelABC.apply_gpp_conversion_factor - this conversion is now internal to PhenologyNew.
• Add code to prevent unsupported combinations of FaparLimitationNew and PhenologyNew methods.
• Probably more from writing the user demonstration docs!