Update open_dataset and dataset_keep_vars() to limit dataset to only 1 variable

[tomvothecoder]

Is your feature request related to a problem? If yes, Please describe

The typical workflow is to work on a single variable at a time. XCDAT operations are performed on 1 variable in the Dataset, so it makes sense to limit the Dataset to 1 variable (and bounds)

This inherently makes the Dataset a CDAT TransientVariable, which is a variable with bounds.

Describe your proposed solution

1. Update xcdat.dataset.open_dataset() arg keep_vars to var (type str)
2. Update xcdat.dataset.dataset_keep_vars() arg keep_vars to var (type str)
3. Remove code that transforms the input to a list of str.

Describe alternatives solutions you've considered

Additional context