Allow to modify tags directly from Patient/Study/Series/Instance objects

[gacou54]

Currently, we have to modify the tags with the client:

import pyorthanc

client = pyorthanc.Orthanc(...)
client.post_studies_id_modify(...)

Would be neat if this would work

patient = Patient(...)
patient.modify()

study = Sudy(...)
study.modify(...)

...

[gacou54]

Done in #36