Locit should use the pointing table to determine Antenna pointing direction

[vsmagalhaes]

Currently Locit uses the source table from the cal table as a proxy to determine antenna pointings.
This is done so that the inputs to extract_locit are simply the user parameters and the cal table.
Dealing with this issue will entail:

• Add a parameter to extract_locit, the ms from which the cal table is derived.
• Use the pointing table of the ms as the actual pointings.