Tweak get_param a bit
Add get_params function

This is like get_param, but if there are multiple values, gets them
all rather than just the first one.
Add cat_lines function
Use cat_lines rather than simple concatenation
Change when we copy PBRT reference output to the log directory

Only copy if the PBRT reference output comparison failed.
Implement "compare_with" test parameter

This [possibly-multiple] parameter specifies another test scene file
to compare results with.