add reactions as children of rxno:addition

[StroemPhi]

We have identified the following five reactions to be children of the rxno:addition class:

• Thiol-yne
• Mukaiyama hydration
• Favorskii reaction
• Diimide reduction
• acetalization

These will be added to the branch add-reactions-as-children-of-rxnoaddition to be merged into the upstream repository later.

[StroemPhi]

We needed to copy "addition reaction" from MOP to RXNO before being able to add children to that class in 082b7e2

[StroemPhi]

As we forgot to also import the children of MOP_0000642, but more importantly, as it is better to directly import the whole MOP, we did so in dd66397.
We think the direct import of MOP is better than the previous manual imports of MOP terms into RXNO, as MOP provides the general terms that are needed in RXNO.

[hujo91]

#4 adresses the problem of confusion between MOP:0000629 & RXNO:0000246