What does this PR do?

This is a prototype to showcase how the current Stats object could be modified to internally have a switch between NSLC and SourceID system. This is an alternative approach with trying to have a minimal amount of outward facing class/hierarchy changes, in comparison to changing the class structure/hierarchy like prototyped in #3550

>>> from obspy.core import Stats
>>> header_sid = {'namespace': 'FDSN', 'network': 'BW', 'station': 'MANZ', 'location': '', 'band': 'B', 'source': 'H', 'subsource': 'Z'}
>>> stats = Stats(header=header_sid)
>>> print(stats)
       namespace: FDSN
         network: BW
         station: MANZ
        location: 
            band: B
          source: H
       subsource: Z
            type: SID
       starttime: 1970-01-01T00:00:00.000000Z
         endtime: 1970-01-01T00:00:00.000000Z
   sampling_rate: 1.0
           delta: 1.0
            npts: 0
           calib: 1.0
>>> print(type(stats))
<class 'obspy.core.trace.Stats'>
>>> stats.to_NSLC()
>>> print(stats)
         network: BW
         station: MANZ
        location: 
         channel: BHZ
            type: NSLC
       starttime: 1970-01-01T00:00:00.000000Z
         endtime: 1970-01-01T00:00:00.000000Z
   sampling_rate: 1.0
           delta: 1.0
            npts: 0
           calib: 1.0
>>> print(type(stats))
<class 'obspy.core.trace.Stats'>

Why was it initiated? Any relevant Issues?

MiniSEED v3 uses the newly introduced FDSN Source Identifier which differs from the "traditional" NSLC (net/sta/loc/cha) way to label data.

Compare #3550

PR Checklist

• Correct base branch selected? master for new features, maintenance_... for bug fixes
• This PR is not directly related to an existing issue (which has no PR yet).
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