Release type handling

[boferri]

The mapping of the MBZ 'release types' seems to be a bit difficult, since they are directly related to MBZ 'release groups' which are mapped to mo:SignalGroup [1] instances. This class is on the expression level. However, the mo:release_type [2] property currently relates to mo:MusicalManifestation [3], i.e. to instances that are on the manifestation level, e.g., mo:Release [4] instances. I'm currently unsure whether we should switch the domain of mo:release_type to mo:MusicalExpression [5], or whether I should create a mapping for mo:Release instances that are related to their release types.
Currently, I would prefer the second one.

[1] http://musicontology.com/#term_SignalGroup
[2] http://musicontology.com/#term_release_type
[3] http://musicontology.com/#term_MusicalManifestation
[4] http://musicontology.com/#term_Release
[5] http://musicontology.com/#term_MusicalExpression