Currently only PGA(3) is supported, but that's because the parser is
rather simple (recursive descent with a lame lexer), but it works well
enough for playing with geometric algebra without having to recompile
every time.
This gives the resultant point the correct sign. Though the projective
divide would take care of the sign, this makes reading the point a
little less confusing (still need to sort out automatic blade reversals
for the likes of e31).
