Convenience functions for effective carrier capture coefficient and defect-limited carrier lifetime

[kavanase]

It would be quite useful to users if some convenience functions for computing the effective carrier capture coefficient (given one or multiple levels with different electron/hole capture coefficients for a defect – using the formula in Alkauskas et al.), and for obtaining the defect-limited carrier lifetime (also given in that paper, just N_defect $\times$ C_tot) were added to this package.

These are useful values to report in publications, and the code to compute them is essentially trivial.