Data Fragment Selectors

[floWetzels]

This PR extends the profile and mapping to cover the integration of data fragment selectors according to Issue #21:

• the property usageInfo has been added to the profile and mapping
• the decription of the profile for the type Data now mentions the possibility to also describe data fragments via objects of this type

@HLWeil @stuzart Should I also add hasPart to point from files to fragments within themselves or should this only be part of the datamap profile?

[HLWeil]

Hmm, we kind of have

1. align it with the arc-datamap profile
2. align it with the requirements of RO-Crate, where it's probably preferred to have an object representing the full file

Therefore maybe it's better to copy the section from the datamap profile?