fbpic/fbpic: 0.7.1

This is a bug-fix release. It fixes two bugs that were introduced in version 0.7.0: The first bug only affected simulations performed on CPU (not on GPU), and typically lead to spuriously high charge density on the axis (see #191). The second bug affected restarts from a simulation checkpoint, and typically lead to incorrect continuous injection of plasma and/or simulations crashing at restart time (see #192). These two bugs are fixed in version 0.7.1