Add exponential source profile to OSKAR

Spiral star-forming galaxies are the majority of the radio galaxies the SKA will observe. They typically have an exponential rather than a Gaussian profile. This means they appear bigger and less concentrated towards the centre than a Gaussian source and, as a consequence, they fill the sidelobes more. 

Representing all the extended sources as Gaussians in simulations can reduce the validity of specific tests (for example calibration and imaging, source detection).

In the OSKAR sky model input file, an exteded source is listed as:

RA, DEC, Flux, size1, size2, PA

where RA and DEC are the sky coordinates; Flux is the integrated flux, size1 and size2 are the major and minor axis size and PA is the position angle. Currently size1 and size2 can only be FWHM of a Gaussian.

For acceptance we need:

1) A way to specify whether the source is Gaussian or Exponential (for example, by means of a flag number as additional attribute after PA)

2) if Exponential, size1 and size2 in the source description are the half-light radii of an exponential

3) The specified profile is used to draw the source in the sky map