We need an extract API for the agg_curves generated by the ebrisk calculator

[micheles]

Without tags, it should be compatible with the agg_curves generated by event_based_risk calculator. It should also work with insured losses. We must discuss with Paolo and the risk team the use cases. in particular which kind of curves should be displayed together.

TODO:

• [ ] add a clear error when the number of tag names does not match the number of tag values
• [ ] add a clear error when the user passes less tag names than needed
• [ ] add a clear error when passing invalid tag names
• [ ] add a clear error when using tags in event_based_risk
• [ ] add an error if loss_type, absolute or kind are missing
• [ ] add a test with a single tagname (possibly in the server tests)
• [ ] see what happens when a tag contains a tricky character
• [x] make the API compatible with the one for average losses. i.e. /aggregate/avg_losses