Clean up Data Model language to not be library specific

[ethanrd]

[Just wanted to capture this detail here.]

CF has a fairly new section on groups (The NetCDF Data Model: Groups), and it contains the following quote and attribution:

"Dimensions are scoped such that they are visible to all child groups. For example, you can define a dimension in the root group, and use its dimension id when defining a variable in a sub-group."

@lesserwhirls pointed this out awhile back and I'm planning to submit an issue to CF to change this quote because the reference to "dimension ID" is an implementation/library detail which CF works to avoid in favor of sticking to the netCDF data model.

Though the attribution in CF says NetCDF Data Model, the page linked is not a data model page and it mentions other netCDF-C details. So, I then went to look at the NetCDF Data Model page. It looks good in terms of not having netCDF-C details except for this sentence:

Each file begins with at least one group, the root group. The user may then add more groups, receiving a new ncid for each group created.

which needs to be reworked to drop the reference to the netCDF-C specific "ncid".