Finite-cone NumPy J and D factors

See Units, shapes and model contracts for units, shapes, domains and backend boundaries, and Quickstart for executable minimal examples.

jfactor(halo, dist_pc, roi_deg, *[, ...])	Return integral rho^2 ds dOmega in GeV^2 cm^-5 (gamma must be < 1.5).
dfactor(halo, dist_pc, roi_deg, *[, ...])	Return integral rho ds dOmega in GeV cm^-2.
C_J	Convert a numerical J factor from Msun^2/pc^5 to GeV^2/cm^5.
C_D	Convert a numerical D factor from Msun/pc^2 to GeV/cm^2.