Remove explicit 'invalid value' imputation

[diehlbw]

Overview

Several reports of confusion around -1 imputation for "invalid" predictions.
It only made complete sense in loose typed binary classifications, so was going to start causing issues with stricter datatypes as more variation is used.

Description of changes

Remove the actual -1 imputation (based on time comparisons)
Update the validation check to directly make the time comparison, specifically
Changed pdh.valid_event -> filtered frame to pdh.is_valid_event -> mask
Use pdh.is_valid_event more comprehensively instead of oneoff checks

Author Checklist

• Linting passes; run early with pre-commit hook.
• Tests added for new code and issue being fixed.
• Added type annotations and full numpy-style docstrings for new methods.
• Draft your news fragment in new changelog/ISSUE.TYPE.rst files; see changelog/README.md.