Revise snowball correction step to handle partially-saturated grouped data

[stscijgbot-jp]

Issue JP-3835 was created on JIRA by David Law:

As discussed by https://jira.stsci.edu/browse/JP-3805, introduction of flagging for cosmic rays saturating in grouped data meant that the snowball algorithm no longer effectively found all snowballs.  A workaround was implemented for build 11.2 to instead flag cosmic rays saturating in grouped data as DO_NOT_USE instead of SATURATION, but ideally the snowball detection code should be updated to work with saturated grouped data too.

Setting to 'housekeeping' priority, as this does not have scientific impact.