Plan with RDA question format crashes

[nicolasfranck]

Please complete the following fields as applicable:

What version of the DMPRoadmap code are you running? (e.g. v2.2.0)

v3.0.1

** Behaviour:**

When a plan phase with an RDA question is opened,
the plan crashes.

Possible cause:

It uses the form helper former_for which expects all attributes mentioned
to be present in the model. But standards is not part of the model Answer.
So either hidden_field_tag should be used, or a method with that added
to the model (which would an odd name). By the way: f.hidden_field thinks
the second argument is a hash of options to be merged..

I'll submit a PR