Currently ARF is being overestimated in eSASS srctool command

[jessicapilling]

In srctool, we currently provide an image to the extmap argument. This image is the combined image from combined eventlists - so it covers a large area.

The way srctool makes it's arf is by dividing this area - given by the extmap argument, into a grid. For each timestep, the effective area is calculated for each point on the grid. Then an overall arf, specific to the area we give to the extmap argument is produced.

Since we give a large image to scrtool currently, we will be overestimating the arf for each spectra.

Solution:
We need to instead mask out the non-cluster parts of the image and give this masked image as the extmap argument.

The size of this mask is up for debate - I think making a radius slightly bigger than the user provided outer_radius argument to srctool_spectrum should be fine.