Bring ESMX_Data enhancements back into ESMF

[theurich]

This PR is close to a full rewrite of the ESMX_Data implementation. The bulk of this implementation was developed under the NRL NEPTUNE and ESPC projects, where we use ESMX to build, run, and validate components and applications. Key aspects of this work include:

• Support for multiple geom definitions
• Each field to specify the associated geom, can be different for different fields
• Grid definitions to support periodic options (for global grids), as well as support local area grids
• Better diagnostic output options for fields, with options to diagnose, issue warnings or errors
• Use of the standard NUOPC Diagnostic attribute to control field data output to NetCDF.
• More generic options for field data initialization and advance, with support for use of import field data in simple arithmetic expressions for feedback
• Support switching between Mediator and Model timekeeping mode. This affects time stamping of export fields. It affects where in the run sequence the ESMX_Data instance can/should be placed.

This PR supersedes #483, and closes #521.

The README.md associated with this PR provides a useful high-level overview of the features provided by the proposed ESMX_Data implementation.

The associated nuopc-app-prototypes PR is esmf-org/nuopc-app-prototypes#16