Confirm usefulness of TileServer.modify_metadata() and whether to a) delete or b) keep and document

[rwood-97]

Describe the solution you'd like
TileServer.modify_metadata() is a helper function which currently is not used within code or shown as example throughout documentation.

Can we decide:

1. whether it is useful
2. whether to add to updated download code
3. whether to add to documentation (assume yes, if yes to both above)

Thoughts:

• Users can select map sheets by querying metadata. Is it neccessary to actually remove items from metadata?
• Did this method come about due to overlapping map sheets?

[rwood-97]

TileServer class has been superseded by SheetDownloader in PR #95