bottom_mask: Create a mask identifying the deepest cell without missing values..
This converts a dataset to a mask identifying which cell represents the bottom,
for example the seabed. 1 identifies the deepest cell with non-missing values.
Everything else is 0, or missing.
This will only work for single file datasets.
The method will modify the dataset in place, so make a copy if you want to keep the original.
If you wanted to create a mask identifying the bottom, you would do this: