Add dialects to mavloss and improve mavtomfile.py

Forgot that I had the dialect commit in my master branch. It's a trivial one, however, so hopefully it can be merged in with this PR.

This also changes the struct field naming for generated m-files to be `{columns:[], data:[]}` instead of `{heading:[], data:[]}`, as heading isn't really the correct term to use here and columns is much more descriptive.