Move missing schema alert from engine to client

having the alert in engine makes it unskippable, which
disrupts the utility of setting yes=True like on every
other command that asks for confirmation. Moving it to
client makes it so that y=True bypasses the alert.
model['time'] is not always initialized
numpy.histogram breaks on nans
Merge pull request #52 from fiendish/master

skippable missing schema alerts and initialize time and fix HIST on nans