Threaded grids, and avoiding duplication

[RemiLehe]

This PR is only a suggestion -- feel free to reject it.

The aim of this PR is to implement the threaded and non-threaded version of simple functions (in this case, particle push and a few grid function) without duplication. This is done by redefining prange and a JIT decorator njit_parallel in the file threading_utilities.py:

• if threading is available, prange is the real prange and njit_parallel includes the parallel flag.
• otherwise, prange is simple range and njit_parallel is simply njit.

Using this trick, some of the grid function were implemented with threading (but others remain to be implemented, such as shifting of the grid in spectral space). I also removed the duplicated particle pusher functions. Eventually, we could probably avoid duplication in the gathering function and deposition functions as well.

Drawback: If the user has numba 0.34 installed, but chooses use_threading=False, threading will still be used for the grid functions, and for the particle pusher. (On the other hand, this is already the case for the FFT...) I am not sure what the best solution to this is, so far. Any suggestion is welcome. Or if you are fine with this drawback (at least for now), this can be merged.
*The user now chooses whether or not to use threading by setting the environment variable FBPIC_DISABLE_THREADING to 1. Therefore I removed the argument use_threading of the Simulation class.

Remains to be done:

• Although the tests should pass with numba 0.33 (because of the checks that were added), they do not pass currently. This should be maybe investigated. This is now fixed.
• Profiling: checking that this speeds up execution, and find which remaining routines should be threaded. Done: the plot below shows the scaling (without the compilation and initialization in "others"). The curve "others" does not significantly go down, which means that other grid methods need to be threaded. In particular, the numpy calculation shift_grid_spectral need to be replaced by threaded numba calls. I can do this in a future PR.