minimal refactoring to use numba - plus some numba decorators
* Numba's jit'ed class cannot reference functions defined later: moving donorcell_op on top
* defining and using float64_1d decorator
autojit'ing functions, moving them in the proper order to avoid
the 'cannot reference functions defined later' problem
Introduced timing and duration option
removing slices and using manual jit instead of autojit