Fixed the Full Age Spectrum (FAS) creatinine equation (egfr_fas_cr()) for ages over 40: the creatinine-ratio exponent is now correctly -1 (a plain reciprocal) as published in Pottel et al. (2016), rather than -0.88. This changes returned eGFR values for patients older than 40.
egfr_ekfc_cr(), egfr_ekfc_cys(), and egfr_ekfc_cr_cys() gain an optional q argument (q_cr / q_cys for the combined equation) to supply a population-, assay-, or individual-specific reference Q value. The default (NULL) continues to use the built-in EKFC reference Q, so existing code is unaffected.