Add bottom layer in the CVMix module and clean up the Stokes drift mo… #234

Job	Run time
conda-forge (ubuntu-latest)	1m 52s
gfortran (9, ubuntu-latest)	3m 55s
conda-forge (macos-latest)	2m 10s
gfortran (10, ubuntu-latest)	3m 59s
conda-forge (windows-latest)	3m 27s
gfortran (11, ubuntu-latest)	4m 8s
gfortran (12, ubuntu-latest)	4m 13s
gfortran (13, ubuntu-latest)	7s
gfortran (7, ubuntu-20.04)	4m 7s
gfortran (8, ubuntu-20.04, -DFABM_USE_CONTIGUOUS=OFF)	3m 54s
	31m 52s

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