add numba dependency

Created by: zeffii

adds numba to possible dependancies

• tested numba install.
• show snlite example of @njit function caching
• Ready for merge.

python features supported in njit (no python mode) : https://numba.readthedocs.io/en/stable/reference/pysupported.html

support for classes https://numba.readthedocs.io/en/stable/user/jitclass.html (jitclass) numba 0.53 https://numba.pydata.org/numba-doc/0.6/doc/extension_types.html (0.60.. not a default pip install)