AxisType: add flag for continuous vs. discrete

For some axis types, it makes sense to interpolate between values, since they are intended to represent continuous axes. For others, they may be discrete—e.g., if each sample along that axis is a different computed feature. It would be nice if the `AxisType` interface had `isDiscrete` and/or `isContinuous` methods reporting that.

Start by locating the AxisType interface and its implementations in the imagej-common codebase, then review any related tests for axis behavior. Decide from the existing type definitions whether the requested continuous/discrete reporting can be specified consistently. Done means the interface and applicable implementations expose behavior that distinguishes continuous from discrete axes, with tests covering the result.

Written by the indexing model from the issue text.