Add 'nodes' as supported indexable element for IJK grid properties

[andy-beer]

The PropertyCollection class does not currently handle 'nodes' as an indexable element for grid properties. This capability should be added for IJK grid properties.

Note that the shape of the points array (nodes) is different depending on whether an IJK grid is faulted (has split pillars) or not. Calling code would have to ensure that properties assigned to nodes are correctly set up, which is non-trivial when split pillars are present.

[andy-beer]

An upcoming piece of work may also require 'nodes per cell' indexable elements for grid properties.