Apply @njit to interp and mlinterp

[oyamad]

interp and mlinterp now return something other than None.

I do not know what this line was doing:

interpolation.py/interpolation/multilinear/mlinterp.py

Line 226 in 705cbce

aa = args[0].types

See the change in 7003f0f

[albop]

@oyamad : Had to go back to an older version of numba to be reminded of what was going on.
The code in @generated_jit was called with the type of its arguments, same as @overload. But the treatment of *args was different. With @generated_jit , the type of args was for instance (UniTuple(Array(float64, 1, 'C', False, aligned=True), 3),) , which includes the type of a tuple of the types passed as arguments. With overload you just get a tuple of the types of each components of args. So all perfectly logical and I can merge this PR.