Trailing slashes in paths break the train switch

Problem: A command such as:

scripts/larvatagger.jl train path/to/backend path/to/data/ mytagger

or:

scripts/larvatagger.sh train path/to/data/ mytagger

fails because of the end slash in path/to/data/.

Proposal: The end slashes could be automatically stripped.

Acknowledgement: Peter Hague for the original bug report.