adding get_position API call

this code adds the get_position() API call, as requested in #105 by @davidhoness and @asb.

This approach has code duplication in api.cpp where there are separate get_x_position() and get_y_position methods, as I wasn't sure about how to return a tuples through the C++/Python interface. I would appreciate if someone (e.g. @Veedrac or @bencatterall) can fix this.