Refactor away instances of OpenStruct

[marcoroth]

StimulusReflex relies on OpenStruct in both the Dataset and the Element models. We should stop using OpenStruct altogether as its use is discouraged.

This was initially raised in #695 and the workaround was addressed by #697, but we should implement the suggested longterm solution proposed in #695 which is to remove all uses of OpenStruct in the gem.