New options for Externals.cfg

[jedwards4b]

Summary of Issue: enhancement request

In Externals.cfg it would be nice to add a feature for automatically handling dependancies between externals repos. For example pop and mom both require CVMix so we might have

[CVMix]
tag = 534fc38
protocol = git
repo_url = https://github.com/CVMix/CVMix-src
local_path = libraries/CVMix
required = False

[mom]
tag = 243bf86c
protocol = git
repo_url = https://github.com/ESCOMP/MOM_interface
local_path = components/mom
externals = Externals.cfg
required = True
requires = CVMix

[pop]
tag = f4c7f4d
protocol = git
repo_url = https://github.com/ESCOMP/POP2-CESM.git
local_path = components/pop
externals = Externals_POP.cfg
required = True
requires = CVMix

So that CVMix is automatically checked out only if mom or pop is checkedout.

But in the above pop and mom are always checked out, so the second part of this enhancement request is for a command line option and new field in Externals.cfg to
allow getting subsets of components so for example --app aquaplanet would get you just the components required for a cam aquaplanet case, this would be noted in Externals.cfg with
[app_lists]
aquaplanet = cime, cam, cice
atmideal = cime, cam
land = cime, clm, mosart