fbpic/fbpic: 0.9.1

This is a minor release, which includes miscellaneous improvements to fbpic: Bug fixes: The restart mechanism now works with ionizable particles. (See #237) For multi-CPU/GPU simulations using the Galilean scheme, the stencil extent and corresponding number of guard cells is now properly calculated. (See #240) In addition, various improvements have been made to the documentation. (See #239 and #240.) Finally, the charge/current deposition is now more efficient on CPU, when multiple species are used (See #234