Add LILAC

Add LILAC: The Lightweight Infrastructure for Land-Atmosphere Coupling. This
infrastructure consists of two major pieces:

(1) A lightweight coupling infrastructure built on top of ESMF that makes it easier for
atmosphere models to call CTSM directly, rather than using the hub-and-spoke architecture
that is used by CESM.

(2) A set of python-based tools for building CTSM and creating its runtime inputs when
running in an atmosphere model via LILAC. Although these tools are built on top of cime,
details of the create_newcase / case.setup / case.build process are hidden from the user,
because many of the aspects of this workflow don't make sense in the LILAC context.

So far we have used LILAC to couple CTSM to WRF. There are plans to use the same
infrastructure to couple CTSM to other regional atmosphere models.

Documentation of LILAC is provided in
https://escomp.github.io/ctsm-docs/versions/master/html/lilac/index.html (though there are
still some missing sections), as well as in various presentations on the wiki
(https://github.com/ESCOMP/CTSM/wiki/Presentations).

There have been many contributors to the development, testing and documentation of LILAC;
chief among them being Mariana Vertenstein, Negin Sobhani, Bill Sacks, Joe Hamman, Sam
Levis, Mike Barlage and Dave Lawrence.