CESM is comprised of a number of different components that are
developed and managed independently. Each component may have
additional 'external' dependancies and optional parts that are also
developed and managed independently.

The checkout_externals.py tool manages retreiving and updating the
components and their externals so you have a complete set of source
files for the model.

checkout_externals.py relies on a model description file that
describes what components are needed, where to find them and where to
put them in the source tree. The default file is called "CESM.xml"
regardless of whether you are checking out CESM or a standalone
component.

checkout_externals requires access to git and svn repositories that
require authentication. checkout_externals may pass through
authentication requests, but it will not cache them for you. For the
best and most robust user experience, you should have svn and git
working without password authentication. See:

    https://help.github.com/articles/connecting-to-github-with-ssh/

    ?svn ref?

NOTE: checkout_externals.py *MUST* be run from the root of the source
tree it is managing. For example, if you cloned CLM with:

    $ git clone git@github.com/ncar/clm clm-dev

Then the root of the source tree is /path/to/cesm-dev. If you obtained
CLM via an svn checkout of CESM and you need to checkout the CLM
externals, then the root of the source tree for CLM is:

    /path/to/cesm-dev/components/clm

The root of the source tree will be referred to as ${SRC_ROOT} below.

To get started quickly, checkout all required components from the
default model description file:

    $ cd ${SRC_ROOT}
    $ ./checkout_cesm/checkout_externals.py

For additional information about using checkout model, please see:

    ${SRC_ROOT}/checkout_cesm/README

or run:

    $ cd ${SRC_ROOT}
    $ ./checkout_cesm/checkout_externals.py --help