Enable multiple laser pulses

[MaxThevenet]

This PR enables multiple laser pulses. Files Laser.H/cpp are renamed MultiLaser.H/cpp, handling multiple laser pulses. The MultiLaser class holds a vector of Lasers, similar to beams or plasmas, with 1 notable difference: the actual data is stored into 1 single array for all lasers, and this array is in MultiLaser. Laser is almost an empty class. In the future, this will be generalised and each Laser instance will have a separate array. Important point: lambda0 is a property of MultiLaser rather than Laser, so we can solve for all lasers at the same time.

Change along the way:

• w0 is not a scalar (previously 2 scalars for x and y but we asserted they were equal). Very easy to change.
• The input parameters are now similar to beams and plasmas, see below. We got rid of the use_laser that @SeverinDiederichs disliked so much :).

The PR is unnecessary long, the content of MultiLaser is just the one from Laser, except for the constructor and ReadParameters. Unfortunately, git mv did not help and git is being confused here.

lasers.names = laser laser2 # deactivated with no_laser
lasers.lambda0 = .8e-6

laser.a0 = 1
laser.position_mean = 0. 0. 0
laser.w0 = 2.*kp_inv
laser.L0 = 2.*kp_inv
laser.focal_distance = 0.001

laser2.a0 = 2
laser2.position_mean = 0. 0. -4*kp_inv
laser2.w0 = 4.*kp_inv
laser2.L0 = 1.*kp_inv
laser2.focal_distance = 0.0005

On Juwels GPUs: top is laser_real, bottom is Ez, for a laser propagating upwards in a plasma. All looks fine.

[SeverinDiederichs]

Thanks for adding this so quickly! Looks fine to me. Let's discuss details offline and potentially fix via follow up PRs

[SeverinDiederichs]

Should we maybe make this queryWithParser and set a default name of "no_laser"?
Otherwise, we break every existing input script. Could be fine of course, but maybe we need to be clearer in the documentation about it.

[MaxThevenet]

Good point. Just for laser? Or we could change it for beams, plasmas and lasers, to make them consistent.