Add skimage deps to preinstall_requirements

* preinstall_requirements: add networkx and pillow
Add pyyaml to requirements.txt
initial, possibly broken, resized image writer

* timestream/manipulate/pipecomponents.py: ResultingImageWriter: add the
	ability to resize images before writing
Fix undefined variables bug in ResFeatureWriter

@Joelgranados please confirm

* timestream/manipulate/pipecomponents.py: Fix what looks like an
  undefined variable bug in the ResultingFeatureWriter class
Always use PCEx exceptions in pipecomponents

* timestream/manipulate/pipecomponents.py: ResultingImageWriter: use our
  custom exceptions, not built in ones.