added example ECEF-TOPO conversion to docstring of `UVData.antenna_positions` 	modified:   uvdata.py
added example ECEF-TOPO conversion to docstring of `UVData.antenna_positions`
	modified:   uvdata.py

HERA-Team/pyuvdata,HERA-Team/pyuvdata,HERA-Team/pyuvdata,HERA-Team/pyuvdata