Change units of PSF table about gamma-astro-data-formats

[cdeil]

The units of the PSF table should be sr^-1 which is the SI unit of a solid angle (see e.g. https://en.wikipedia.org/wiki/Steradian). Square degrees is a non-SI unit

We're using deg in many other places, so I don't see how using deg^-2 could be bad.
@jknodlseder - Is deg also not an SI unit and you want to get rid of it?

But gtpsf uses sr^-1, so for consistency, I'm 👍 to change the psf_table spec to use sr^-1.
I can do the change in the spec.

@lmohrmann or @dltiziani Can you make the change in the HAP exporter?

[cdeil]

@dltiziani - Another thing to fix in the HESS exporter is this:

In [1]: from astropy.table import Table

In [2]: t = Table.read('datasets/hess-crab4-hd-hap-prod2/run023400-023599/run023523/hess_psf_table_023523.fits.gz')
WARNING: UnitsWarning: '1/(deg^2)' did not parse as fits unit: Numeric factor not supported by FITS [astropy.units.core]

We had this issue before ... the problem is the "1/" in the string ... the solution is to write "sr^-1", i.e. don't have a "1/" in the unit string.

[cdeil]

I've implemented this change in #55. Closing this issue now.