Refactoring of the .testing Makefile

Primarily changes to improve coverage reporting, but some other changes
as well:

* New separate rules for `codecov` and `report.cov`

* REPORT_COVERAGE was split into two flags:
  * DO_COVERAGE: Enable code coverage reporting
  * REQUIRE_COVERAGE_UPLOAD: If true, error if upload fails

* We now report codecov upload failures in the log, but only raise an
  error (i.e. nonzero return code) if REQUIRE_COVERAGE_UPLOAD is true.

* GitHub Actions now only reports failed uploads to CodeCov for pull
  requests.

* Separate FCFLAGS_FMS flag to control FCFLAGS for FMS builds
  - also renamed the old FCFLAGS_FMS to FCFLAGS_DEPS, which was
    previously used to pass the flags to the FMS library/modfiles

* $(DEPS) was dropped, and we just use `deps` now.

* Updated the header instructions

* README update

* Codecov.io and GitHub Actions CIs were updated to support changes

.codecov.yml

@@ -8,7 +8,3 @@ coverage:
             default:
                 threshold: 100%
                 base: parent
-comment:
-    # This is set to the number of TCs, plus unit, but can be removed
-    # (i.e. set to 1) when reporting is separated from coverage.
-    after_n_builds: 9

.github/workflows/coverage.yml

@@ -3,15 +3,15 @@ name: Code coverage
 on: [push, pull_request]
 
 jobs:
-  build-test-nans:
+  build-coverage:
 
     runs-on: ubuntu-latest
     defaults:
       run:
         working-directory: .testing
 
-    env:
-      REPORT_COVERAGE: true
+    #env:
+    #  REQUIRE_COVERAGE_UPLOAD: true
 
     steps:
     - uses: actions/checkout@v2
@@ -26,10 +26,26 @@ jobs:
       run: make -j build/unit/MOM6
 
     - name: Run unit tests
-      run: make unit.cov.upload
+      run: make run.cov.unit
 
-    - name: Compile MOM6 with code coverage
+    - name: Report unit test coverage to CI (PR)
+      if: github.event_name == 'pull_request'
+      run: make report.cov.unit REQUIRE_COVERAGE_UPLOAD=true
+
+    - name: Report unit test coverage to CI (Push)
+      if: github.event_name != 'pull_request'
+      run: make report.cov.unit
+
+    - name: Compile ocean-only MOM6 with code coverage
       run: make -j build/cov/MOM6
 
-    - name: Run and post coverage
-      run: make run.cov -k -s
+    - name: Run coverage tests
+      run: make -j -k run.cov
+
+    - name: Report coverage to CI (PR)
+      if: github.event_name == 'pull_request'
+      run: make report.cov REQUIRE_COVERAGE_UPLOAD=true
+
+    - name: Report coverage to CI (Push)
+      if: github.event_name != 'pull_request'
+      run: make report.cov