Add hardcoded element type list as a parameter

[nchenche]

Each read element type of a gff is tested against the list ["chromosome", "region", "match"] to check if the element must be considered or not.

It would be more flexible to make this list as a parameter with the same default values.

For instance:
orftrack --get_element_type_considered --> returns ["chromosome", "region", "match"]
orftrack --delete_element_type_considered "match" -> returns ["chromosome", "region"]
orftrack --add_element_type_considered "any-long-overlapping-region-tag" -> returns ["chromosome", "region", "any-long-overlapping-region-tag"]