Fix folia2text for large number of folia files

The folia2text script crashed when run for a large number of folia files
(>33). The error message was the same for when a very large (or
mulitple) xml file(s) without the option huge_tree=True. However, this
option was set correctly, and the error still occurred. To fix this
error, the script was rewritten to process a single folia file (instead
of for all files in a directory). Also a bash script was added to run
folia2text for all files in a directory.
Move functionality to emotools/folia_helpers

Move functionality to add entities and write xml to file to the
emotools/folia_helpers module.
Update add_entity for use with annotation objects
Update kaf2folia script

The kaf2folia script takes 2 arguments: the folia xml file and a (matching)
tag file. The folia xml file is overwritten with the updated xml. This
script must be called by a bash script.
Add bash script to call kaf2folia script