Update of terminologies

The terminologies, or specifically the terminology definitions should be updated and made a bit more generic.
Example, the definition of Subject currently states:
"_The investigated experimental subject (animal or person). May contain the Cell and Preparation sections as subsections._"
The first part is awesome, the second sentence is unnecessary and too specific.
Subject section could also contain a Training or a Surgery subsection.

Updating the terminologies should depend on the outcome/solution of #138. If the G-Node OEN can be linked and the definition with this automatically set we only have to correct/maintain one source instead of two.