[PDP] Add term diff features, and improve enrollment year

[bdewilde]

changes

• adds a new family of cumulative features referred to as "term diffs", which compute term-over-term differences for a configurable set of columns; I tried to keep this minimal, and cover three basic indicators: “participation” (num courses taken), “progression” (num credits earned), and “performance” (avg course grade)
• (re-)implements logic to compute start/end dates for terms and cohorts, and uses term+cohort start dates to compute a new "year of enrollment" feature that should align with how PDP computes it in other, similar fields
• adds cleanup logic to nullify column values based on *_term_X, in the same way as already implemented for *year_Y features
• adds a minimal types.py module, in which we can place shared types; starting out with just TermType

context

Continuing to add more info for model training, as possible. All of the year-of-enrollment stuff came out of this Slack thread

Follows PR #14

questions

• Is the generic term diff logic correct?
• Is the new term dt / enrollment year logic correct? Would you like me to drop the old enrollment year feature in favor of the v2?