Curation of metadata created by mergeDims

[davestanley]

When the user calls mergeDims, it creates the following metadata. This metadata might be used later on when unmergeDims is subsequently called.

obj.axis_pr(dims2merge(1)).axismeta.premerged_values = temp;
obj.axis_pr(dims2merge(1)).axismeta.premerged_names = mergednames;

Presently, operations such as obj.subset or obj.merge (or even subsequent calls to mergeDims) will not actively curate this metadata.

We should update the getters and setters (or other data that interacts with obj.axis) in order to actively curate these or, alternatively, return a warning to the user when they become misaligned.