Add SVO data because SVO is down

[hugobuddel]

See http://svo2.cab.inta-csic.es/theory/fps3/fps.php "We are having some technical problems and our services will be unavailable for some days. "

Should fix #184

[hugobuddel]

This PR adds many of the SVO files (at least the ones used by the test suite) to a data directory, and packages that directory within the ScopeSim package.

ScopeSim first looks in that data directory for the SVO files, and only requests them for SVO if they cannot be found.

This data directory might not be the best way to solve this problem. Nevertheless, I'm merging this now, because at the moment ScopeSim is effectively broken for new users who do not have their astropy cache filled. Plus, other PR's are now blocked, such as #187

We can discuss whether we want a better solution later.

For example, it is still worthwhile to have a test that actually tries to download a file from SVO, to check whether that works. That test could then perhaps be marked xfail or something.