Add manage externals to access MMM-physics repo

[weiwangncar]

TYPE: enhancement, no impact

KEYWORDS: manage_externals, some physics for tropical suite

SOURCE: internal

DESCRIPTION OF CHANGES:
Add the use of manage_externals tool to access physics in MMM-physics repository. The physics we access are part of the 'tropical' suite: YSU PBL, revised MM5 surface layer, WSM6 microphysics, and new Tiedtke scheme. It also accesses GWDO option 1 routine. These modules have been residing in phys/physics_mmm/ in 4.6. Instead of copied files in this directory, we now use manage_externals to access these modules from MMM-physics repository.

Will need to update the tags specified in Externals.cfg before final release.

LIST OF MODIFIED FILES:
A arch/Externals.cfg
M phys/Makefile
D phys/physics_mmm/bl_gwdo.F90
D phys/physics_mmm/bl_ysu.F90
D phys/physics_mmm/cu_ntiedtke.F90
D phys/physics_mmm/module_libmassv.F90
D phys/physics_mmm/mp_radar.F90
D phys/physics_mmm/mp_wsm6.F90
D phys/physics_mmm/mp_wsm6_effectRad.F90
D phys/physics_mmm/sf_sfclayrev.F90
A tools/manage_externals/

TESTS CONDUCTED:

1. Successfully compiled the code on Derecho.
2. Are the Jenkins tests all passing?

RELEASE NOTE: Add manage_externals tool to access physics modules in MMM-physics git repository.

[weiwangncar]

The regression test results:

Test Type              | Expected  | Received |  Failed
= = = = = = = = = = = = = = = = = = = = = = = =  = = = =
Number of Tests        : 23           24
Number of Builds       : 60           57
Number of Simulations  : 158           150        0
Number of Comparisons  : 95           86        0

Failed Simulations are: 
None
Which comparisons are not bit-for-bit: 
None