Allow `title_quantiles` inheritance from `quantiles` only if `len (quantiles)==3`

[sybreton]

title_quantiles inherits quantiles values only if len(quantiles)==3, update accordingly documentation for show_title and title_quantiles.

This should restore backwards compatibility with corner <= 2.2.1, before the argument title_quantiles existed, where quantiles lists of arbitrary length could be passed together to show_title=True.

See issue #237 and #193.

Close #237