- Convert any labels specified on the command line using `safe_float()` just like the labels that are read by all of the FeatureSet reader classes. This ensures that something like `-L 0 1` is interpreted correctly.

- Right now, the input and output file need to be specified _before_ any optional arguments which is unintuitive. This commit makes it such that the files also require flags but they are required.

- Add new tests to check that the label conversion works correctly.
- Update existing tests to use the new command line argument flags.

- Update the command line arguments.
- Add a warning explaining the backwards incompatibility and changes that will be coming up in v2.5.