fbpic/fbpic: 0.15.0 Perfectly-Matched Layers in radial direction

This release adds the possibility to add Perfectly-Matched Layers in the radial direction (see #417). As part of this change, the user is now expected to pass a dictionary for the argument boundaries of the Simulation object (instead of a single string) ; for instance: boundaries={'z':'open', 'r':'reflective'}. The PML in the radial direction are activated by passing 'r':'open'.

In addition, several others changes have been included in this release:

• It is now possible to use FBPIC with numba 0.46 and cupy 7 (see #414).
• The default phase of the FlattenedGaussianLaser has been updated. (see 411)
• A new laser profile has been added: FewCycleLaser which is well-adapted for tightly-focused, short laser pulses (see 403).