Reorganise ObjectClassification

[mswoodburn]

The ObjectClassification class was originally intended to handle informal (and potentially hierarchical) classifications, such as the high-level classification used by the SYNTHESYS+ Collections Digitisation Dashboard (https://rebrand.ly/synth-cdd) across biological and geological domains. It's gained a number of taxonomic and other properties that don't really fit this use, and might be better moved. Some suggestions:

• Move period and material to ObjectGroup
• Use the Taxon class from DwC and suitable instead for taxon terms identified
• Add properties for informal hierarchy of classification to ObjectClassification
• Reconsider the class name, and make sure the definition is clarified

[mswoodburn]

I've carried out the actions above. Still some thought about whether the class name is the most appropriate though, I think.