Sync Gtilde speedup

[tibaldo]

This simple mod speed up the calculation of synchrotron spectra by 40%. See discussion in #135.

[zblz]

Can you rebase on master so that only the last commit (03a4ae4) remains in the PR? The first two are already in master and the merge commit introduces conflicts in travis.yml.

[tibaldo]

There is a conflict I cannot resolve in any way, I'll start clean with another branch.

[zblz]

I see you created a new PR, but for the record this could have been resolved with git rebase -i origin/master and then dropping the first couple of commits. After rewriting the branch you have to force push to update

[tibaldo]

I tried to do this, but the rebase was failing due to non-mergeable conflicts in .travis.yaml that I could not figure out how to resolve (although they seemed relatively trivial).
Anyway, sorry for the example of bad practises, but I got frustrated and it was way faster to do the new PR.