run "cfchecks" from PATH lookup instead of hard-coding the path
change behaviour of wrapper so that --update_only must be given when not checking any files (which also changes default value for the -t flag); this syntax change allows any invocation without args to fail with a (brief) usage message
A method was being used to determine the type code of a variable which worked for a FileVariable but did not work for an AxisVariable (specifically var.dtype.char), so it fell over in one case when checking the type of the time axis data. Add a method to get the type code for various relevant object types, and call this instead of testing obj.dtype.char throughout.
1-line bugfix in opt parsing
update URLs to point to cfconventions.org; the same URLs are defined both in the library and in the wrapper, and there could be an argument for not coding them into the wrapper, but will leave this aspect for now