Fixes for pdf

- Change urban figure from gif to png (gif doesn't work with pdf output)

- Remove some broken latex coding from the References section; it
doesn't seem like this was accomplishing anything before (based on
looking at an old version of the pdf, generated Feb 2019)

Author: billsacks

Externals_CLM.cfg

@@ -16,7 +16,7 @@ required = True
 local_path = doc/images
 protocol = git
 repo_url = https://github.com/ESCOMP/CTSM-doc-images.git
-hash = 0f039e7a043fabfce91ddf4f41186a3014603aac
+hash = 9e12e5531d8cd3f2e9c6a0154dba8289ffd32c7f
 required = False
 
 [externals_description]