document when predicted distribution is erased

[mathematicalmichael]

the current default mode (for backwards compatibility) is distribution-inversion (._setup[iteration]['col'] = False), col for collapse (QoI), which does not impose any relationship between observed/predicted.
Therefore, predicted should only be "erased" (set to None) if we're in data driven mode, with a warning thrown to the logger. If not in data-driven mode, do not raise any warning.