Make --slspec/--json flags optional

By estimating the multiband factor directly from the ".eddy_movement_over_time" file, we can completely ignore the --slspec/--json flags.

For single-band data the --slspec/--json flags are still used to estimate which excitations did not cover the brain (and hence have no information to drive the slice-to-volume registration). If not provided, an interleaved acquisition is assumed (odd slices first).