Calculations for previous are in ImagePotHandler.

* timestream/manipulate/plantSegmenter.py (segment): Remove any handling of
the previous image in the segmenter class
(ImagePotHandler.mask): Handle the previous pot here.
Restructure mask method in ImagePotHandler

* timestream/manipulate/plantSegmenter.py (mask): We remove 2 levels of
indentation effectivelly increasing readability.
Fix bug. Replace mask with the same size.

There was an error when the previous mask was of different size than the
current one.
* timestream/manipulate/plantSegmenter.py (mask): Change size of prevous mask
to fit the current size.