This commit introduces the first set of
changes for Query-Driven Visualization (QDV)
of AMR datasets. The changes are as follows:

(1) The base reader implements RequestInformation()
    and defines a pure abstract method FillMetaData()
    to be implemented by concrete classes.

(2) Concrete AMR readers implement FillMetaData() which
    essentially creates a vtkHierarchicalBoxDataSet with
    the uniform grids and AMR boxes but with no data, e.g.,
    density etc.

(3) The metada is published in the output information object
    for downstream modules.