fix landsat 4/5 fft filename bug

[forrestfwilliams]

This PR makes a small change to the file path that FFT-filtered images are written to. Instead of writing them to /working/directory/{BASE_NAME}_fft.tif, we now write them to /working/directory/fft/{BASE_NAME}.tif. This ensures that routines that rely on string indexes of standard Landsat image names work correctly.

[jtherrmann]

Looks good, just wanted to confirm there are no tests to update?

[forrestfwilliams]

No not currently, but the fact that we have no tests for this functionality may be a bigger issue.

[forrestfwilliams]

@jhkennedy and @jtherrmann, currently the create_fft_filepath function creates the directory in which the data is saved. I'm concerned this may be an inappropriate side effect for this function. Thoughts?

[jtherrmann]

@jhkennedy and @jtherrmann, currently the create_fft_filepath function creates the directory in which the data is saved. I'm concerned this may be an inappropriate side effect for this function. Thoughts?

@forrestfwilliams Yeah it may be better to create the directory right after this line, since it should be the same directory for both calls of the create_fft_filepath function, right? So no need to attempt to create it twice.

needs more work (see comments)