Add support for non-uniform X axis in array data

It is now possible to construct AnalysisArrayData objects that have a
non-uniform X axis.  AnalysisDataAverageModule can use this to provide a
more meaningful X axis in the output plot in case the values correspond
to, e.g., a set of atoms.

Change-Id: I578f6944d4ff51c315d068b011b705be2a4be00a