filename bug in sfrdata.py

I pulled today which brought me up to 2e44c8e. In sfrdata.py, at line 1548 looks like `obs_input_filename` is defined but on the next line `just_the_filename` is passed to `options`. However, unless `options` is `None`, `just_the_filename` is not defined. I think the fix is just to use `obs_input_filename` but wanted to make sure.