Investigation subclasses should be deprecated

[mcourtot]

These have been frequently confused by users as indicating types of restriction on data access, whereas they were only intended to describe types of research without any commitment to data use. They are a remnant from early versions of DUO where we had intended to describe subtype of research.

This ticket proposes to deprecate them to prevent further confusion.

note for implementation: this also means updating preferred root terms files for display