[TYPO3-mvc] Where do helper classes go?

[Sy Moen]

I have "trails" as coordinate sets in my model, but in order to display them
in certain cases, the coordinate sets must undergo heavy transformation. I
cannot just plug the data into a template. For instance gpx or (ouch) MS
Streets and Trips data has to be transformed to a ubiquitous format via an
external (system) utility.

I assume this is a "Service", right? Like the mp3 meta extractor, or a
pdf2txt class... those would be services I assume.

Maybe someone knows of an extension that does this already that I can look
through?