Allow User to use cquad integration directly

[paulrogozenski]

Currently, the user can only choose between qag and spline integration when computing $C(\ell)'s$. It would be good to have a high-accuracy/high-fidelity version accessible to the user by calling cquad integration directly (instead of reverting to this when qag roundoff fails). Having this integration scheme can help resolve integration errors when integrating over non-smooth distributions, when kernel overlap is small, and when taking numerical derivatives of $C(\ell)'s$.