use bcdata for tsa and bec

[boshek]

Closes #62

This removes all code to handle tsa and bec differently. Rather these are just now part of the typical make_shortcuts workflow.

[ateucher]

I think this looks great, thanks @boshek! This might also be a good time to do a little cleanup on layers_df.csv. As far as I can tell we are not using the local column any more... and it would be good to fully populate the uses_shortcut column so it's all T/F, with no NAs to reduce ambiguity.