TRIM interface

Add interface (TrackTrim) for importing output files generated using the TRIM program (http://www.srim.org/). Like the "TRIMCAT" interface in classic Garfield, it reads the TRIM output file EXYZ.txt'. There are some small differences between the implementation in TrackTrim` and classic Garfield:

• TrackTrim reads multiple ion trajectories from the file and cycles through them when generating tracks (whereas "TRIMCAT" reads a single ion trajectory at a time).
• TrackTrim uses directly the energy steps given in EXYZ.txt, whereas "TRIMCAT" does a rebinning.

Possible improvements:

• Add option to read COLLISON.txt instead of EXYZ.txt.
• Handling of multiple layers.
• At the moment, the cluster position corresponds to the end of a step. It could instead be in the centre, or one could randomize it, or have multiple clusters per energy step.
• Instead of requiring the work function and Fano factor to be set by the user, TrackTrim could take it from the Medium.