Implement new tracking pipeline

[AngelFP]

This PR brings in major changes to the way in which Wake-T works internally. In particular, it redefines the way in which the tracking of particle bunches in a field is performed. Currently, these changes apply to the PlasmaStage and its subclasses (i.e., anything which is not a TMElement). The new implementation, including all the changes listed below, results in up to a x4 speedup of the code. It does not change anything in the user API and it only adds a few new optional parameters.

The main change to the working principle of Wake-T is the addition of a new Tracker class (#76) that takes care of orchestrating the evolution of the particle bunches and fields, and generating the diagnostics. This is done in a more general and extensible way compared to the old _track_numerically method in the PlasmaStage. It also supports new features. Among others, tracking of multiple beams simultaneously and the use of adaptive time steps.

The new Tracker moves the simulation in time by determining what should be updated next (pushing a particle bunch, updating the fields, or generating diagnostics), and continues until the specified final time. As an example, a simulation with one particle bunch with adaptive time step, one NumericalField and openPMD diagnostics would look like this:

Bunch: x-x-x--x---x-----x------x--------x---------x----x|
Field: x-------x-------x-------x-------x-------x-------x|
Diags: x-----------x-----------x-----------x-----------x|
Time:  ------------------------------------------------>| End

where the x indicate the points in time where the updates should be made.

As part of this redesign, the implementation of the fields as well as of the beam particle pushing have also been updated:

• Fields: a new base Field class has been implemented from which AnalyticalField (for simple fields based on analytical expressions) and NumericalField (for fields requiring more expensive numerical solutions to be calculated) are derived. This replaces the old Wakefield class. All wakefield models are now subclasses of AnalyticalField or NumericalField .
• Beam particle pushing: the whole process of particle pushing has been redesigned so that the Tracker can be in charge of pushing the beams. This has also been a good chance to implement the RK4 pusher in a much more efficient way, and has allowed the implementation of the new Boris pusher (Implement Boris pusher. #66).

In addition, this PR includes the new features from #77 and #79, which implement a new plasma pusher and bring performance improvements to the Baxevanis wakefield model.

The full list of changes is the following:

• _track_numericaly method replaced by new Tracker class.
• Define new Field base class and the derived AnalyticalField and NumericalField classes.
• Implement Baxevanis and fluid models as NumericalFields.
• Implement analytical wakefield models as AnalyticalFields.
• Avoid instantiating new arrays in the gather methods. Instead, store gathered field in already existing arrays.
• Treat E and B fields separately, no longer as W (solves Treat trasverse E and B fields separately #49).
• New and much more efficient implementation of the RK4 pusher.
• Implement Boris pusher for particle bunches (Implement Boris pusher. #66).
• Move beam pushers to dedicated folder.
• Add bunch_pusher attribute to all plasma elements. Possible values are 'rk4' and 'boris'.
• Expose tracking time step as dt_bunch in all plasma elements (solves Add option for setting tracking time step #53).
• Implement new Tracker class #76
• Implement 5th order Adams-Bashforth pusher #77
• Improve performance of Baxevanis wakefield model #79