Fix for Jenkins TDS test failures for GRIB with unusual mixed time intervals

[ethanrd]

Description of Changes

Fix failure in Jenkins TDS tests on some GRIB files with interesting mixed time intervals. Code to generate time coordinate variables from time intervals wasn't handling case where strict-monotinicity adjustments needed to be made in first timestep.

PR Checklist

• [x] Link to any issues that the PR addresses
• [ ] Add labels
• [x] Open as a draft PR until ready for review
• [x] Make sure GitHub tests pass
• [x] Mark PR as "Ready for Review"

[ethanrd]

This PR fixes a problem with PR #1072 which addresses the 1D time coord system case in issue #1060.

[tdrwenski]

Sorry I didn't see it before, but I just saw two more tests that may be failing related to the time intervals on the netcdf-java job on jenkins. They have the tag NeedsCdmUnitTest so aren't being run on PRs

[Test Result](https://jenkins-aws.unidata.ucar.edu/job/netcdf-java_maint-5.x/153/testReport/) (2 failures / +2)
[ucar.nc2.dt.grid.TestGridSubsetCoordinateSystem.testGridDomain[/share/testdata/cdmUnitTest/thredds-test-data/local/thredds-test-data/cdmUnitTest/gribCollections/rdavm/ds627.1/yearPartition-1979.ncx4]](https://jenkins-aws.unidata.ucar.edu/job/netcdf-java_maint-5.x/153/testReport/junit/ucar.nc2.dt.grid/TestGridSubsetCoordinateSystem/testGridDomain__share_testdata_cdmUnitTest_thredds_test_data_local_thredds_test_data_cdmUnitTest_gribCollections_rdavm_ds627_1_yearPartition_1979_ncx4_/)
[ucar.nc2.iosp.grib.TestIntervalsTimeCoords2D.checkTimeIntervalCoordinates[/share/testdata/cdmUnitTest/thredds-test-data/local/thredds-test-data/cdmUnitTest/tds/ncep/RUC2_CONUS_20km_pressure_20100509_1300.grib2]](https://jenkins-aws.unidata.ucar.edu/job/netcdf-java_maint-5.x/153/testReport/junit/ucar.nc2.iosp.grib/TestIntervalsTimeCoords2D/checkTimeIntervalCoordinates__share_testdata_cdmUnitTest_thredds_test_data_local_thredds_test_data_cdmUnitTest_tds_ncep_RUC2_CONUS_20km_pressure_20100509_1300_grib2_/)

The second one may just need to be updated because it checks what the bounds are.

[ethanrd]

Thanks Tara! @tdrwenski - I'll take a look.