first steps toward fast hets run: adding parameter fast_parse
add tests for git lib check if a file has changed
add git lib file changed checking function
add another test for git lib file changed check
delegate file changed check from model to lib
fast parse only if the ontology is not at the current state
Merge branch 'staging' into 504-hets_fast_run
..and fixed bug "Hets.parse(..., structure_only: false)"

Conflicts:
	app/models/ontology_version/parsing.rb
Merge pull request #630 from ontohub/504-hets_fast_run

Enable structure only parsing for hets, fixes #504.