Rename Number typedef in libagg to AggNumber

The typedef of double to Number in libagg conflicts with the Number
class in libgambit.  This renames the typedef to avoid the clash.
Link libagg into Python extension.

Add the files in libagg to the list of files for building the Python
extension.  This means that AGG files can now be read into a game
object.  Methods on the AGG representation are as yet untested.