prototype numba version of simulator functions

The longer term plan here is to move away from cython in an attempt to become more modular.

The **deliverable** for this issue is a working version of the basic `ddm` simulator that uses numba, and times against the current cython based implementation that underlies call to the `simulator()` function.

Start by locating the simulator() entry point and the current Cython implementation it uses; no specific files are named in the issue. Trace the basic ddm simulator, then prototype its Numba version and compare timings with the Cython implementation. Done means the Numba version works and the timing comparison is available.

Written by the indexing model from the issue text.