fastmath keyword argument for jit and boost (for Numba)

[paugier]

See https://numba.pydata.org/numba-doc/dev/user/performance-tips.html#fastmath

Simple to implement:

• add the argument in

  • https://github.com/fluiddyn/transonic/blob/master/transonic/aheadoftime.py#L101
  • https://github.com/fluiddyn/transonic/blob/master/transonic/justintime.py#L188

• improve the Numba backend https://github.com/fluiddyn/transonic/blob/master/transonic/backends/numba.py