Pointing list stuff & airy disk gain pattern

Dearest Sahm,

I think I'm finally starting to figure out my way around github... Anyway, the edits I've made so far include:

- Capability to set self.gainpat = 'gaussian' or 'airy'. This can be done within the generate() parameters (for generating a population) or inside a survey parameter file (so that the gain pattern is used for a given test survey). At the moment, 'pattern' is an input parameter to the Survey init method, which seems a bit cludgy, but maybe I/we can think of a better way to do that if you would prefer changing this before merging.

- Two extra attributes for the the Pointing class: 'self.gain' and 'self.tobs'. This allows the user to set these parameters independently for a given beam (which is useful for something like a multibeam receiver or cases where tobs changes inside of a given survey). These attributes come into play every time a new offset is calculated between pointing and pulsar to keep track of corresponding central gain and integration time values for that pointing. Also, I've changed the expected # columns for 'pointingslist' from 2 (gl/gb or ra/dec) to 4 (gl/gb/gain/tobs or ra/dec/gain/tobs) if 'pointfname' is provided.

I think that's everything. Let me know if you have questions/comments!

Joe