Upgrade to openPMD-viewer 1.1.0

This PR updates VisualPIC to take advantage of the new features in the latest openPMD-viewer.

Changes

Thanks to the new openPMD-api backend, non-HDF5 files are now supported.
The access to any openPMD data is now handled by the new DataReader in openPMD-viewer, which simplifies the implementation in VisualPIC.
The backend used by the openPMD DataReader (either 'h5py' or 'openpmd-api') can be manually specified when creating the DataContainer.
Some custom VisualPIC features have now been directly implemented in openPMD-viewer (https://github.com/openPMD/openPMD-viewer/pull/306, https://github.com/openPMD/openPMD-viewer/pull/307). This greatly simplifies the some parts of the code.
Added initial support for data in cylindrical geometry.
Fields in 3D Cartesian geometry are now always returned with [x, y, z] axis order. If the original data has a different order, then the axes are rearranged with np.moveaxis.
FolderField and ParticleSpecies now have a direct mapping between time steps (iterations) and data files.
The axes arrays in field metadata used to have one more element than they should. This has now been fixed.
Other bug fixes.
Updated README.md with new installation instructions.