Ground Control Points interface

Currently it’s a bit difficult to create a ground control points file. Ideally starting from the concepts available in GitHub - OpenDroneMap/posm-gcpi: Ground Control Point interface (GCPi) for OpenDroneMap, a GCP interface should be integrated in WebODM that makes it easy for people to create a GCP file.

A really, really cool feature would be the automatic detection of GCP markers across images. Many JS frameworks such as could allow this to be implemented directly in the browser.