Allow alternate grid mappings for geometry containers

Fixes #156

conformance.adoc

@@ -544,16 +544,16 @@ attributes.
 
 *Requirements:*
 
-* One of the dimensions of the data variable with geometry must be the number of
-geometries to which the data applies.
 * The type of the **`geometry`** attribute is a string whose value is the name of
 a geometry container variable. The variable name must exist in the file.
-* The geometry container variable must hold **`geometry_type`** and
-**`node_coordinates`** attributes.
+* The geometry container variable must hold **`geometry_type`**,
+**`geometry_dimension`**, and **`node_coordinates`** attributes.
 * The only legal values of geometry_type are **`point`**, **`line`**,
 and **`polygon`** (case insensitive).
 * For a line **`geometry_type`**, each geometry must have a minimum of two node coordinates.
 * For a polygon **`geometry_type`**, each geometry must have a minimum of three node coordinates.
+* The **`geometry_dimension`** attribute must name a dimension that is shared by
+the data variable. This dimension represents the total number of geometries.
 * The type of the **`node_coordinates`** attribute is a string whose value is a
 blank separated list of variable names. All specified variable names
 must exist in the file.
@@ -567,6 +567,11 @@ associated with the container.
 container variable or its parent data variable has a **`nodes`** attribute, then the **`nodes`** 
 attribute must be a string whose value is a single variable name. The specified variable must be
 a node coordinate variable that exists in the file.
+* If **`grid_mapping`** and **`coordinates`** attributes are carried by the
+geometry container variable and the **`grid_mapping`** attribute names a
+different grid mapping variable than the **`grid_mapping`**  attribute on the
+associated data variable, then the variables indicated by the **`coordinates`**
+attribute must not have a **`nodes`** attribute.
 * The geometry node coordinate variables must all have the same single dimension,
 which is the total number of nodes in all the geometries.
 * Nodes for polygon exterior rings must be put in anticlockwise order (viewed from above)
@@ -576,6 +581,8 @@ of parts in all the geometries.
 * When more than one geometry instance is present and the **`node_count`** attribute on the geometry
 container is missing, the geometry type must be **`point`** and the node coordinate dimension size 
 must be the same as the number of geometry instances.
+* The variable indicated by the **`node_count`** attribute must use the same
+dimension as specified by the **`geometry_dimension`** attribute.
 * If a **`part_node_count`** variable and a **`node_count`** variable are present for a given geometry
 container, then the sum of **`part_node_count`** values must equal the sum of **`node_count`** values.
 * If the **`interior_ring`** attribute is present on the geometry container, then the **`part_node_count`**