ign.hh: Does not expose all functions consumed by `ign sdf`?

At present (f94626a889), `libsdformat` only exposes these functions in `ign.hh`:
https://github.com/osrf/sdformat/blob/f94626a8898b0a2898ae65892d76fd1031456133/src/ign.hh#L30-L37

However, `ign.cc` defines more (`cmdDescribe`, `cmdPrint`):
https://github.com/osrf/sdformat/blob/f94626a8898b0a2898ae65892d76fd1031456133/src/ign.cc

These are consumed by the intermediate Ruby script:
https://github.com/osrf/sdformat/blob/f94626a8898b0a2898ae65892d76fd1031456133/src/cmd/cmdsdformat.rb.in#L150-L158

It would be nice if these were exposed via the header file.

In https://github.com/robotlocomotion/drake/pull/13767, I want to use the `check` and `print` functionality to lint and convert models.

(I'm OK-ish with it being an unstable private API, but if it's there, would be nice to have it be consistent?)

Compare the declarations in src/ign.hh with the implementations in src/ign.cc and the calls from src/cmd/cmdsdformat.rb.in. Determine how cmdDescribe and cmdPrint should be exposed consistently, then verify that the ign sdf check and print functionality can use the exposed API.

Written by the indexing model from the issue text.