All optional child nodes should be flagged explictly with an attribute mandatory="false"

Every child node that is potentially OPTIONAL should explicitly be flagged with an attribute mandatory="false".

We should also decide what to do for the mandatory children. There are two possibilities:

1. either we explicitly flag ALL mandatory child nodes as mandatory="true"
2. or we do NOT add ANY mandatory="true" anywhere but assume this as the default value when the mandatory attribute is missing (We don't mind to go for this second solution, but then it should be checked carefully that all cases where the mandatory attribute is currently missing indeed correspond to a case where mandatory="true". This is not the case right now. We found cases of nodes where the mandatory=... attribute is absent but the node was either mandatory of optional.

More on this issue can be found in point 4.A of the file metamodel/experiment1.txt