POP is released via subversion, but it includes external code that is released
via git. Rather than relying on subversion's svn:externals property, POP now
uses the manage_externals utility from

https://github.com/NCAR/manage_externals

Since this is not tied to subversion, after checking out POP users must run

    $ ./externals/manage_externals/checkout_externals

Note that CESM also uses manage_externals and will run checkout_externals when
checking out POP, so this step is only necessary for non-CESM checkouts of POP.
