Resolve "FAR and FRR thresholds are computed even when there is no data support"

Closes #27 (closed)

I have updated and corrected the threshold computation for FAR and FRR thresholds. Now, when there is no possible way to reliably compute a threshold for desired far or frr values, the far_threshold and frr_threshold functions return NaN.

The threshold computation has changed a little, but all related tests still pass. I have added new tests that test the NaN return values for reasonable test cases.

mentioned in commit 5ace8e26

merged

@bob: I'll merge this as it looks OK-ish. Extensive testing with the next release will tell us better.

mentioned in issue #27 (closed)