Change MetadataKey type/category/name getters to start with 'Get'

Merged Morten Ofstad requested to merge feature/morten.ofstad/FixMetadataKeyGetters into master

Changed MetadataKey getters to start with 'Get' to be consistent with getters for VolumeDataAxisDescriptor/VolumeDataChannelDescriptor and also generate read-only properties for those in the Python API.

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