Extension of calcFrPrp() but with ignore argument.
Use ignr to ignore certain tips from calculation. For example, if any of tips
are extinct you may wish to ignore these.
References
Isaac, N.J.B., Turvey, S.T., Collen, B., Waterman, C. and Baillie, J.E.M. (2007).
Mammals on the EDGE: conservation priorities based on threat and phylogeny. PLoS ONE, 2, e296.