Templatize spreadinterp and more cleanups

[mreineck]

This is a massive change to spreadinterp.cpp, both reordering the functions in the file to avoid forward declarations, and templatizing everything in there.

I'd be happy to get this in as soon as possible, but I appreciate that this will be very hard to review...

[mreineck]

There are now no more doubly-compiled sources in the library (there are still doubly-compiled tests however). The current state is not pretty, since I had to make a few compromises to arrive at this point, but from now on things should improve again.
Still, before continuing, I'd like to have some feedback whether this has a chance to be accepted.

[mreineck]

Current test failures seem to be related to network problems on the CI servers ... they failed to download FFTW.

[mreineck]

OK, I'll stop myself here and wait for the review.

Don't get me wrong: there is still much to be done to really gather the benefits of this restructuring ... but all these subsequent changes will be more or less localized, and they will be much more fun than it was getting to the current state.

My hope would be that this can be merged before any other of the open PRs, because incorporating any change on master into this PR will be extremely painful. Of course I'll be happy to help with adjusting the conflicts I cause in the other PRs!