straightening.cache file is output to the working directory, even when -ofolder is used

[joshuacwnewton]

For scripts that have an image-straightening step (e.g. sct_register_to_template, sct_label_vertebrae), we allow re-use of straightened image files to save time. To facilitate this, we use a straightening.cache file.

In sct_register_to_template, if -ofolder is used, the 3 straightened image files will be output to -ofolder, but the straightening.cache file will not. Instead, it will be created in the PWD.

By comparison, sct_label_vertebrae will correctly output all 4 straightening files to the -ofolder path.