mda.fetch_pdb() to generate Universe from Protein Databank structures

[orbeckst]

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Is your feature request related to a problem?

We used to have mda.fetch_mmtf() to get a structure from the PDB but with the demise of MMTF, this was removed around release 2.6 or so.

Describe the solution you'd like

It would be very convenient to have mda.fetch_pdb(PDB_ID) to create a Universe from the entry in the Protein Databank with ID PDB_ID.

Describe alternatives you've considered

VMD (load molecule), pymol (fetch) and Chimera (fetch by ID) can do this.

Alternatively, keep using this bash script or wget/curl.

Additional context

We will need the mmcif reader #2367 .

See also #3377 (fetch from alphafold database).