Flesh out placeholder and missing test cases

[kgdunn]

Migrated from TODO.md (Tests section).

Items covered

• Flesh out partial / placeholder test bodies: R cross-checks, SPE-vs-SIMCA-P comparison, Plotly assertions, etc. (tests/test_multivariate.py)
• Investigate the sequence that yields Sn = 0 despite variability. (tests/test_univariate.py)
• Complete the robust-case test. (tests/test_univariate.py)
• Address the bare # TODO. (tests/test_univariate.py)
• Add a NaN-at-start example and a NaN-in-warm-up example. (tests/test_monitoring.py)
• Add pandas-Series input tests, handle NaN in vectors, and add shape edge cases (5x2 / 7x1, 5x4 / 5x1, 5x5 + intercept failure). (tests/test_regression.py)

Note: code line numbers in the old TODO.md are stale; locate the code by symbol name.