Implement tracing along either axis of 2D image

For simplicity's sake, it was decided to initially only support dispersion along the horizontal axis. However, there is enough need already to support dispersion along either axis that this should be added sooner rather than later. The dispersion axis will be specified when the trace is created and `Trace` classes will have `disp_axis` and `crossdisp_axis` attributes.

Start by locating where traces are created and reviewing the Trace classes. Follow how the current horizontal-only dispersion is represented, then determine the changes needed for either axis, including the disp_axis and crossdisp_axis attributes. Done means traces can be created with either dispersion axis and both orientations are supported.

Written by the indexing model from the issue text.