Image solving requires a catcache directory to function

[simonrw]

The image offset shifts are calculated from the cached wcsfit downloaded catalogues. On the first run this is not present, so the program fails.

[simonrw]

I've added a terrible hack: when the cached data is not found, wcsfit is run to build an initial solution and the cache. The central shifting is then applied after the cache is present.

This obviously runs the limiting performance part of the code twice which is not a good idea.