Add function for scipy.integrate.quad

Users of numerical integration don't have a nice lowlevelcallable interface with Numba right now. See:

See https://docs.scipy.org/doc/scipy/reference/ccallback.html

Start by reading the SciPy LowLevelCallable documentation linked in the issue, with particular attention to scipy.integrate.quad and its callback requirements. Identify the appropriate entry point for a new helper in llc-tools; done means users have a Numba-compatible low-level callback interface for numerical integration.

Written by the indexing model from the issue text.