Determine directory structure for aerosol code

During the review of #398, @nusbaume originally wrote:

> Instead of having the CCPP-layer in a ccpp subdirectory, I wonder if it better matches other schemes to have them be at the microp_aero layer, and then have all of the core functionality in a subdirectory instead? This would make the directory layout similar to all of the mmm schemes, as well as external schemes like rrtmgp, and eventually clubb.
>
> Of course I realize changing the directory layout could impact CAM, so if that's the case then I am happy to just make an issue and hold off until we have a reason to modify CAM and the aerosol microphysics again. Thanks!

Start by reviewing issue #398 and the proposed alternatives for the aerosol directory layout. Compare the current CCPP-layer placement with the microp_aero/core structure used by the MMM schemes and external schemes such as rrtmgp and clubb. Confirm the impact on CAM before deciding on a directory structure.

Written by the indexing model from the issue text.