utilities.create_subsetdb throw error for data types that are not string

Admittedly a very small thing but it would be great for the utilities.create_subsetdb function to explicitly throw a type error when the provided healpix values in the list are not cast as strings.

Took a while to figure out what was going wrong.

Start at the utilities.create_subsetdb entry point and trace how the provided healpix values are handled. Confirm the expected behavior for non-string values and verify that they raise a TypeError rather than failing later with a less explicit error.

Written by the indexing model from the issue text.