Initial version of CLM5.0

This is the initial science version of CLM5.0.

Developments for CLM5.0 build on the progress made in CLM4.5. Most major components of the model have been updated with particularly notable changes made to soil and plant hydrology, snow density, river modeling, carbon and nitrogen cycling and coupling, and crop modeling.
The updates to CLM science are documented here:

https://escomp.github.io/ctsm-docs/doc/build/html/tech_note/Introduction/CLM50_Tech_Note_Introduction.html#clm5-0

This version has production versions of CLM5.0 CMIP6 land-use change files for 1850, historical, and 2000. It has preliminary versions of CMIP6 forcing files for Nitrogen deposition and population density. Further updates are needed for prescribed aerosols, and future scenarios.

Spunup initial conditions are provided for several CLM physics and forcing combinations from simulations run at 1-degree resolution (fv 0.9x1.25 with g1xv6 ocean mask).

Update initial condition files, update fates, new CMIP6 pop-dens, more lnd_tuning_mod options, updated ndep for fully coupled

Update the initial condition files for clm4_0, clm4_5, clm5_0 at f09_g16 for both CRUNCEpv7
forcing and GSWP3v1 forcing. Update FATES with science change to science-1.4.1_api-3.0.0.
New CMIP6 population density file that has years beyond 1850 (the old one just had 1850, so
was incorrect for historical or present-day). clm4_0 compsets now use CRUNCEPv7 forcing rather than
the CRUNCEPv4 forcing. LND_TUNING_MODE is now used for initial conditions
for different forcing. For fully coupled 1850 cases the ndep file is updated to a preliminary CMIP6
version. Fix the tools testing and build-namelist testing to work in the git checkout.

clm4_5_18_r274: Fix auto-detection of CIME_MODEL in a standalone checkout

The auto-detection of whether CIME_MODEL is acme or cesm is broken in
standalone checkouts of clm4_5_18_r273. (This auto-detection relied on
whether there was an SVN_EXTERNAL_DIRECTORIES file present at the top
level.)

This tag points to a new cime version that fixes this issue.

clm4_5_18_r273: Support a standalone checkout from git

This PR allows a standalone git-based checkout to work. Starting with
this tag, we'll be using git rather than svn for CLM/CTSM development.

The biggest changes here are pulling in manage_externals, which serves
the role of svn externals. After cloning the repository, you can get all
of the necessary externals by running:

./manage_externals/checkout_externals

See the file README_EXTERNALS.rst at the top level of the repository for
more details on using this tool.

Other than that, this PR mostly involves changes to allow you to run
from the new directory structure of a standalone checkout: Now all of
the CLM/CTSM directories appear at the top level of the checkout, rather
than nested under components/clm.

clm4_5_18_r272

Last version working only in subversion. This version does not work in a git checkout!