Added dada2uvfits converter

Added a python script to convert dada files into uvfits files. This is
actually a two step process, and involves calling two C programs:
lconvert: converts from dada into LA (autos) and LC (cross) correlations
corr2uvfits: converts from LA and LC into uvfits.
One of the nice things is that dada2uvfits automatically generates the
required header from the dada filename and optional arguments.