Add labels when plotting channels as func. of t

shown with --legend flag
Beginning of phase rotation stuff

1. Create antenna_functions.py
This contains a simple array class for doing uvw calculations
based on antenna / array co-ords and target ra/dec/times

2. Add option to do phase rotation using this class into
corr_grab_h5.py. Save antenna/array locations into file
as well as a 'phased_to' dataset, which contains the phase
centre for each sample

3. Add antenna/array location stuff to config file

4. Have the AmiDC class instantiate an Array object for
use by other methods as AmiDC.array