utc2et test and kernel checking added

I've added a test for utc2et (my first :+1: ). I based it on the example given in the NAIF docs for the icy version of utc2et. There is a slight difference of c. 5 seconds between the output value given in the icy example and the value SpiceyPy gives for the conversion.

I've also added a check to getkernels so that the kernels are only downloaded if they don't exist in the cwd... saves them being downloaded every time the tests are run.

This is my first contribution to a github project other than my own and my first pull request, so if I have missed a step or left out some vital info. please let me know.

J.