MultipleSeries.pca(): raise on unequal lengths instead of returning None

[vaibhav8a]

Fixes #702.

Problem

MultipleSeries.pca() guarded unequal series lengths with a bare print() and then fell off the end of the function:

flag, lengths = self.equal_lengths()

if flag==False:
    print('All Time Series should be of same length. Apply common_time() first')
else: # if all series have equal length
    ...
    return res

So the caller got None back. The failure surfaced later as AttributeError: 'NoneType' object has no attribute 'screeplot' — far from the actual cause — and the message itself couldn't be filtered, captured by logging, or suppressed the way warnings.warn can.

Change

Raise a ValueError naming common_time() as the fix, and include the lengths that disagreed:

if flag == False:
    raise ValueError(
        "All series must be of the same length to perform PCA; "
        f"got lengths {lengths}. Apply common_time() first."
    )

The else branch is untouched, so the equal-length path is byte-for-byte the same.

Testing

Added TestMultipleSeriesPca::test_pca_t4, which builds a MultipleSeries of unequal length and asserts a ValueError mentioning common_time. Confirmed it guards the change: with the multipleseries.py hunk stashed the test fails with Failed: DID NOT RAISE ValueError, and passes with it applied.

pytest pyleoclim/tests/test_core_MultipleSeries.py — 100 passed.

Scoped to pca() only. The other sites in the #697 survey depend on the unified-policy decision that's still open, so I left them alone; this one is the case #702 calls out as a bug in its own right regardless of how that lands.