Add possibility to add scenario-id to create_scenario

[Charlottevm]

Currently it is not possible to update an existing scenario with an ESDL-file (other than using the create_with_context api). It would be awesome (and eventually necessary) to be able to do this with the create_scenario endpoint as well.

Steps:

• Define steps that need to be taken
• If more time is needed then 0.5 day, report back
• Otherwise, go for it whenever possible! :-)

Follow up issues:

• Re-namespace the API: create and update can be in the same endpoint but should be POST and PUT respectively #95
• Validate the scenario to be updated to match the ESDL (area and end year) #96