Using bathymetry.py create a simple unit test with child grid as a 1:1 sub region of the parent (i.e. longitude, latitude, depth grid points are all colocated)

• Parent dx/dy 1\deg, -20E:10E, 45N:65N
• Child dx/dy 1\deg, -10E:0, 50N:60N
• Depth levels 50:5000:100 using the flat bottom case
• Generate constant variables

Test success: output files match subset of Parent data