Concept and sandbox relationship

[c-martinez]

At the moment, when switching from Zenodo to Zenodo Sandbox, the user needs to update sandbox: true to sandbox: true in the yaml file. But one also needs to update the concept: 1234567 (which could be a common "gotcha").

One suggestion would be to have two separate variables in the yaml configuration:

    concept: 1234567
    concept_sandbox: 7654321

In this way one would only need to change sandbox.

In read_inputs this conditional selection already is made for token, so a similar approach could be taken for concept_id.

This could avoid confusion for users, and facilitate testing on Zenodo Sandbox.