Good to have File I/O reader handle dimensions and attributes

### What is the feature/what would you like to discuss?

Right now the `abstract_netcdf_reader_t` class and its `pio_reader` CAM-SIMA version can only read NetCDF variables. However, several physics schemes would benefit from being able to read NetCDF dimensions and attributes as well.

Given this, it would be good to expand the object's functionality to provide a `get_dim` and `get_att` method, which would help reduce the code burden in the physics schemes themselves.

### Is there anyone in particular you want to be part of this conversation?

@jimmielin (who brought up the need to read in NetCDF attributes)

### Will this change (regression test) answers?

No

### Will you be implementing this enhancement yourself?

Any Software Engineer can do this