Explicit export of Reactions

[HeikoKlare]

Currently, a ChangePropagationSpecification is created for each Reactions segment in the default package mir.reactions.SEGMENT_NAME.
To provide a clean interface to the generated code, we should introduce a kind of export keyword that explicitly marks a segment to be exported and accessed by applications. Therefore, it should be possible to define a package in which the ChangePropagationSpecification for that segment should be created.
We could then disable the generation of ChangePropagationSpecification for each segment per default.

Together with that fix, we should change the default namespace of Reactions and Routines since the term mir is outdated. We could use something generic in our namesapce like tools.vitruv.consistency.*.

[HeikoKlare]

There is another reason to have an export keyword: WIth the new design, we have to define an explicit segment that combines all segments that shall be used in one ChangePropagationSpecification. If there are routines with conflicting names in these segments, they must be imported with qualified names, although this segment does have the purpose to use or even modify routines. Therefore, we should treat the export segments completely different and not allow to define additional reactions and routines there and also not allow to import them anywhere else.