"cellnum" (node_attribute) fix and layer bottom fix requests

I have 3 requested updates:
1. The latest version of SFRclasses allowed self.node_attribute to be labeled in the XML file, but assignment in the exception cause problems in "cellnum" hadn't been generated yet.  I changed the assignment to a boolean false to allow "cellnum" to be generated (with that name), and also added a check for "cellnum" in MFgrid so that it works properly with preproc = false.
2. This version had problems fixing the layer bottoms again.  It appeared to be an indexing issue again.  I reverted back to the latest changes I had from a month ago when Mike and I worked on this.  It ran, so appears to have worked.  (I have additional evaluations to perform)
3. I added a flag so that I could opt-out of generating stream profile plots and PDFs because they take a long time during a debug session.  Could probably be improved, but worked for me (see SFR_main_example.py)