WekaSource: streamline validation mode parsing
WekaSource: no functional change, just cosmetics

Flipping around the test whether input filename has changed,
returning right away if it has not changed.
WekaSource: postpone 'mode' and 'done' control changes by one tick

Only change mode at the tick that actually produces output under
that particular mode of operation, not one tick before.

This allows correct synchronization of modes with related MarSystems
(e.g. a classifier) based on simply linking their 'mode' controls.