Function to return hidden items

[PatrickRWright]

Is your feature request related to a problem? Please describe.
Sometimes during a study, certain fields will be hidden because data should no longer be entered into them (other reasons may be possible too). Its probably useful to have a small helper function which can tell you for which variables this is relevant. i.e. return_hidden_items()

Describe the solution you'd like
return_hidden_items()
Will also needs a check whether hidden fields were exported (i.e. #171).

Describe alternatives you've considered
Putting in in diff_secuTrial() but a separate function is the better solution.

Additional context
#169