Make the size param to ResImgWriter a scalar

I think that we should avoid supporting a list of sizes. If people want
many sizes, they can call the imagewrite component many times with
different values. This solves the issue of how to name many different
sizes.

* timestream/manipulate/pipecomponents.py: (ResultingImageWriter)
	Force the sizes parameter to be a scalar, and rename it to be
	size