Automatically adjust image_length and image_width based on "collect_mode"

[scottyhq]

client.create_tasking_request(collect_mode="stripmap")

Currently retains default values for image_length=5000 and image_width=5000 which is for a standard spotlight product. It would be convenient for the client to automatically set these to image_length=20000 and image_width=5000, which is what seems to happen in the web UI.

[grmpflh27]

create_[tasking|repeat]_request were cleaned up in #96 to align to Capella's tasking on console.capellaspace.com.

I consider this issue to be closed by that - feel free to re-open if you disagree.