Add citation and references for aPower

Added comment in ridefile.cpp to reference the book
and research paper that aPower is based upon:

// Calculate derived data series, including a new metric aPower
// aPower is based upon the models and research presented in
// "Altitude training and Athletic Performance" by Randall L. Wilber
// and Peronnet et al. (1991): Peronnet, F., G. Thibault, and D.L. Cousineau 1991.
// "A theoretical analisys of the effect of altitude on running
// performance." Journal of Applied Physiology 70:399-404