Add a warning of how many GB of data downloading will generate

[rwood-97]

Describe the solution you'd like At the moment, users can download all map sheets from a series if they like but this could generate hundreds of GBs of data. We should add a warning/info message to say approximately how much data it is likely to create before downloading. Maybe with a user input to say confirm and a force argument for if it is used as part of a script.

How this would work:

• work out how many MB 1 tile is
• work out how many tiles it is in total (I think this is possible from GridBoundingBox)
• calculate total size of requested download