Modify diffstar kernel to return log_mah in addition to sfr

[aphearin]

The _lax_ms_sfh_scalar_kern_scan function is the core kernel where a scalar value of diffstar SFR is computed. Currently, this kernel computes log_mah from the diffmah parameters, and then sfr from the diffstar parameters, but the kernel only returns sfr, and so if we need log_mah in addition to sfh then we'd need to recompute log_mah, which is inefficient and clunky. The fix is to have this scalar kernel return both log_mah and sfr, but that will break a lot of downstream code. And so we might should wait on this change until we do some other code restructuring in diffmah such as ArgonneCPAC/diffmah#134 and ArgonneCPAC/diffmah#135.