BUG/TST: Fix investigation_type setter

The investigation_type setter was not working in the PrepTemplate class
because the `_investigation_type_ontology` value was missing when this
object was being instantiated.

I have added 3 tests to exercise this attribute of the class.

Fixes #594
Merge pull request #598 from ElDeveloper/issue-594

BUG/TST: Fix investigation_type setter