CCPP I/O reader: dimension query function (and other size/shape operations)

### Issue Type

Infrastructure Update

### Issue Description

During the review of #504, @nusbaume originally wrote:
> I believe adding a dimension-getting method wouldn't be too difficult. There is also the general work around of just grabbing a variable that has the associated dimension and then just using the size or shape intrinsic functions to get the actual dimension values. So I don't think that particular missing feature would be a dealbreaker.

This will be used to replace many instances of PIO use throughout CAM-SIMA

### Will this change answers?

No

### Will you be implementing this yourself?

Any Software Engineer can do this

Start by locating the CCPP I/O reader and the PIO call sites this change is intended to replace throughout CAM-SIMA. Define the dimension-query and related size/shape operations needed for those replacements, then verify that the reader supports the required use cases.

Written by the indexing model from the issue text.