The ``Dimension'' property is an unsigned integer representing the
dimensionality of the co-ordinates. This has the default value zero,
for unknown or no coordinates, 2 for 2-dimensional co-ordinates (such as
depictions) and 3 for 3-dimensional co-ordinates. This property is
typically set by the file format reader indicating the dimensionality of
the input file (0 for SMILES, 3 for MOL2 and 2 or 3 for MDL SD files
etc...)
