GCPlist cannot contain NaN values.

[jonbrussell]

The instructions in the GCP Editor Pro interface say "Elevation values can be set to "NaN" to indicate no value" and by default when creating a GCP list from "Create GCP File from Map" it includes NaN values. This appears to cause processing to fail. Replacing the NaN value with "0" seems to allow WebODM to process without processing error.

Reference-

https://community.opendronemap.org/t/add-gcp-file-error-cannot-process-dataset/11699/4

[Saijin-Naib]

Anecdotally, I've seen this same failure multiple times in the recent past and setting the elevation to 0 instead of NaN does indeed fix the issue.

I am uncertain if the proper approach lies in fixing how we parse the values, or whether or not, indeed, 0 should be passed instead.

If it is as simple as changing the docs/recommendation, I can address that in a PR.

What do you think, Piero?

[JoeKae]

#35 Adds option to replace NaN with 0 on Z column