Additional relationships on process, file, and windows-registry-key SCOs

["Emily Ratliff" <Emily.Ratliff@ibm.com>]

As our product teams
have started playing with STIX 2.1, they have requested additional relationships
between SCOs.  I filed GitHub issues to cover these requests.
windows-registry-key
https://github.com/oasis-tcs/cti-stix2/issues/225
file
https://github.com/oasis-tcs/cti-stix2/issues/224
process
https://github.com/oasis-tcs/cti-stix2/issues/222

They are straight-forward
and there is precedence for this in the Domain SCO. Since we have to have
another CSD anyway, would it be possible to add these in now?

Thanks,

Emily Ratliff
STSM, Security Architect
IBM Security