Implement relational associations between AttributeMatrices at varying hierarchies

Currently, there is no direct connection between a lower-level AttributeMatrix and a higher-level AttributeMatrix that is an agglomeration of units at the low level. An example is Feature Ids at the element level and the corresponding FeatureAttributeMatrix.

While the Feature (or Ensemble) Ids provide a method of indexing into the higher level AttributeMatrix, there is no direct connection between the various levels. This means that a user has to personally bookkeep the relations in the data structure. A more complete implementation is to subclass AttributeMatrix with specialized classes that take another AttributeMatrix pointer in the constructor.