File format independent data loading

[gwirn]

!! This PR introduces breaking changes !! _It is not possible any more to load multiple trajectories with multiple consecutive calls to PDBData.importpdb()

With this PR MDAnalysis is used to read in one ore multiple trajectory files. This is now done with a single call:

• for one file without a topology file
  • PDBData.import_pdb("path/to/trajectory.pdb")
• for one file with a topology file
  • PDBData.import_pdb("path/to/trajectory.dcd", "path/to/topology.pdb")
• for multiple files without a topology file
  • PDBData.import_pdb(["path/to/trajectory0.pdb", "path/to/trajectory1.pdb"])
• for multiple files with a topology file
  • PDBData.import_pdb(["path/to/trajectory0.dcd", "path/to/trajectory1.dcd"], "topology.pdb")

It still preserves the original functionality of the PDBData class.

Additionally the atom selection has changed - it can still be done via a list of atoms like ["CA", "C", "N", "CB", "O"] but can now also use the MDAnalysis selection language