Parse size in __init__ of ResImageWriter

This implements Joel's suggestion of parsing the size param in __init__
of the ResultingImageWriter, so that it dies while the pipeline is being
set up, not at each timepoint in execution.

* timestream/manipulate/pipecomponents.py: ResultingImageWriter: Use
	__init__ to parse the size param. Also support float/tuple
	directly, not just strings.