TST: add edge case tests for npv and irr (gh-58)

[Eshan-kumar-jain]

Partially addresses #58 by adding edge case tests for npv and irr.

New tests:

• npv: zero rate, all-negative cashflows, single cashflow
• irr: single positive cashflow, zero cashflows in the middle
  (expected value derived analytically rather than hard-coded)

Leaves #58 open, since it covers the other functions too.

While testing I noticed two behaviours that look unintended, so I
did not write tests locking them in:

• npf.irr([]) raises a bare IndexError (from row[0] on an empty row)
• npf.irr([-100, np.nan, 110]) raises LinAlgError from np.roots,
  whereas npv returns NaN for NaN input

Happy to add tests for these once the intended behaviour is decided.

[Eshan-kumar-jain]

@Kai-Striega could you approve the CI workflows when you have a moment? Happy to make any changes.