Previously, coordinate systems in a DataSet simply pointed to field data
specifying the coordinate information (although the ability to get that
back out of the DataSet was missing). This makes sense since point
coordinates are in fact just fields with a particular semantic meaning
to them.

However, there is an issue with this approach. It turns out that there
are special representations that are very common for point coordinates
and very uncommon for other types of fields. For example, a uniform
(a.k.a. regular or image) grid has point coordinates that are easily
derived from the point index, but such fields are quite uncommon
elsewhere.

Representing this kind of structure in the Field list of a DataSet is
problematic. Either all fields have to check to see if they are this
type, which will cause an explosion of unnecessary generated code, or
you will have to actually write out the coordinates in memory, which is
really wasteful but what was done previously.

However, by storing fields representing coordinate systems in a separate
batch, we can use these special types without the stated explosion.