refactoring signor

[EvanDietzMorris]

SIGNOR refactor:

• The main issue was that qualifiers cannot be lists, I changed the single values to strings, and split edges when >1 (anatomical context qualifier)
• It was calling get_latest_source_version in the constructor, we never want to do that but here it didn't even have a purpose
• It was reading/parsing the node mapping file from disk for every single node mapping (that's sloooow and inefficient)
• The mapping file we create was generated in get_data, that should really go in parse_data (get_data should only download source data, any manipulation or parsing of the source that is subject to change needs to go in parse_data or it won't run again with a new parser release)
• The way extractor was used doesn't really use the features of extractor and complicates things some so I just took it out

nitpicky stuff:

• main data file was saved as getLatestRelease.php, changed that to a .tsv file
• flattened unnecessary loop in get_data
• make_datafile() and signor_data.json were a bit ambiguous, changed their names to make_signor_type_map()
• changed get_node() to fix_node_curie_prefix()
• cleaned up the function that returned subject and object ids