allow specification of output measurements

[MatthewWhiting]

This was prompted by ticket #33. It would be nice to have the ability to specify which output columns are written. If the peak column density is included, it will not be of interest a lot of the time. The way around that would be to have an input parameter specify a list of quantities to be written. e.g.

outputParams ra,dec,f_int,w50,col_dens,s/n

This would impact on the output functions -- each parameter would need a check to see whether it appeared in the list before it was written out.