Fixes SyntaxWarning that pops up when plotting sometimes with newer versions of Python:
.../clawpack/visclaw/src/python/visclaw/plotpages.py:605: SyntaxWarning: "is" with a literal. Did you mean "=="?
if plotdata.format is 'forestclaw':
...clawpack/visclaw/src/python/visclaw/plotpages.py:1809: SyntaxWarning: "is" with a literal. Did you mean "=="?
if prefix is 'fort':
Fixes
SyntaxWarning
that pops up when plotting sometimes with newer versions of Python: