Function readPercentileBandOptions

Validates an adapter's percentileBands option.

An entry that is not an object, or names no median, is reported and left out. The rest -- the bands, a title and a name -- is checked by the shared declaration validator, so the levels are held to exactly the rules the co-located block is: fractions, each band straddling the median, and bands that nest. An entry it refuses is left out, and its series are read as the undeclared chart reads them.