added threadsafe to skin and noskin signature files

[jbusecke]

I added the threadsafeuser statement to both signature files. This blocks using the callback function, but since we did not have success with the error messages until now, I hope that this will improve the performance a bit as implied by @rabernat

[jbusecke]

I just tested this PR locally and got some really encouraging results:

This is a soft scaling experiment on my local machine:

Up to 4 threads the relative execution time remains very close to 1! Afterwards it increases almost linearly, which makes sense since my laptop has 4 cores (there are some low energy cores, but I am unsure if these are recruited for stuff like this).

[jbusecke]

Ill merge this now and release, so we can hopefully use this for tomorrows session.