Simplify tower types

[samsrabin]

Description of changes

Simplifies child types of TowerSite (NeonSite, Plumber2Site).

Moving here from the PR on my fork.

Remaining work

• Resolve merge conflicts
• Remove check for valid site type

Specific notes

Contributors other than yourself, if any: None

CTSM Issues Fixed: None

Are answers expected to change (and if so in what way)? No

Any User Interface Changes (namelist or namelist defaults changes)? No

Does this create a need to change or add documentation? Did you do so? No

Testing performed, if any:
As of a91c855:

• All Python unit testing
• All Python style testing
• ./run_neon --neon-sites TOOL --run-type ad should complete without issues (see run_neon not working as expected for AD simulations #2739)
• clm_pymods suite

[samsrabin]

@TeaganKing I moved this here from my repo and got rid of the site type checking as discussed. Could you re-review?

[TeaganKing]

This generally looks good! I will test this out more thoroughly on Thursday.

[TeaganKing]

I ran a few NEON & PLUMBER cases and they both seem to be working smoothly! Thanks for making these changes!

[samsrabin]

Thanks, @TeaganKing!