noresm_v6_cam6_3_123: Add taper fix

[gold2718]

Summary: New version of CAM-Nor FV dycore with extended taper for AM fixer with supporting namelist changes

Contributors: tto061, gold2718

Reviewers: oyvindseland, tto061

Purpose of changes: Add capability for extended taper of FV AM fixer (Issue #95)

Github PR URL: https://github.com/NorESMhub/CAM/pull/96

Changes made to build system: None

Changes made to the namelist:

• fv_am_fix_taper: Added flag to taper the AM fixer
• fv_am_fix_tpr_h: Added height (pressure) at which to begin the taper
• fv_am_fix_tpr_w: Height (pressure) interval for fixer

Changes to the defaults for the boundary datasets: None

Substantial timing or memory changes: None

• Externals_CAM.cfg: New CAM-Nor-physics tag (containing code changes to implement AM fixer)
• bld/namelist_files/namelist_definition.xml: Added new namelist variables (see above)

Tested with aux_cam_noresm, all tests pass

Issues addressed by this PR: closes #95