Improve code documentation for MPAS dynamical core (#356)

### Tag name (required for release branches):

None

### Originator(s):

kuanchihwang

### Descriptions (include the issue title, and the keyword ['closes', 'fixes', 'resolves'] followed by the issue number):

Improve code documentation for MPAS dynamical core.

* Add license header to each module
* Add descriptions to each module
* Keep code comments up-to-date
* Add support for auto-generating API documentation by using Fortran Documenter (FORD)

### Describe any changes made to the build system:

None

### Describe any changes made to the namelist:

None

### List any changes to the defaults for the input datasets (e.g., boundary datasets):

None

### List all files eliminated and why:

None

### List all files added and what they do:

* `A       src/dynamics/mpas/assets/ford-config.md`
  * Add configuration file for Fortran Documenter (FORD)

### List all existing files that have been modified, and describe the changes:

* `M       src/dynamics/mpas/driver/dyn_mpas_subdriver.F90`
  * Add license header to each module
  * Add descriptions to each module
  * Improve code documentation
* `M       src/dynamics/mpas/dyn_comp.F90`
  * Add license header to each module
  * Add descriptions to each module
  * Improve code documentation
* `M       src/dynamics/mpas/dyn_coupling.F90`
  * Add license header to each module
  * Add descriptions to each module
  * Improve code documentation
* `M       src/dynamics/mpas/dyn_grid.F90`
  * Add license header to each module
  * Add descriptions to each module
  * Improve code documentation
* `M       src/dynamics/mpas/stepon.F90`
  * Add license header to each module
  * Add descriptions to each module

### Regression tests:

This is a documentation-only PR. No changes are expected to any existing tests.

Author: kuanchihwang

src/dynamics/mpas/assets/ford-config.md

@@ -0,0 +1,33 @@
+---
+project: MPAS Dynamical Core
+summary: MPAS dynamical core solves the fully compressible non-hydrostatic equations of atmospheric motion.
+author: CAM-SIMA & MPAS Developers
+github: https://github.com/ESCOMP/CAM-SIMA
+website: https://sima.ucar.edu
+src_dir: ..
+exclude_dir: ../assets
+             ../dycore
+output_dir: ./ford
+extensions: F90
+            f90
+fixed_extensions: F
+                  f
+fixed_length_limit: false
+fpp_extensions: F
+                F90
+preprocess: true
+preprocessor: gfortran -E
+display: private
+         protected
+         public
+graph: true
+lower: true
+proc_internals: true
+search: false
+sort: alpha
+source: true
+---
+
+Model for Prediction Across Scales (MPAS) is one of the supported atmospheric dynamical cores in Community Atmosphere Model - System for Integrated Modeling of the Atmosphere (CAM-SIMA). CAM-SIMA is the next-generation atmospheric component of Community Earth System Model (CESM).
+
+This auto-generated documentation provides a developer's guide to the Fortran interface and subdriver of MPAS dynamical core.