[Bf-python] Metaballs rewrite

[Campbell Barton]

Anyone mind if the metaballs api is rewritten?
It dosnt look like many people have been using it, had a look at the 
C/Python and one of addElement's args wasnt evenbeing used and didnt 
match the docs, checks for arg length were missing and in general its 
not up to date with other modules.

Will keep similar names etc- just use getsetattrs and vectors for locations.

- Cam