Good question.  I actually forgot to investigate.  I guess I had been thinking that if a user really cares about the details of how interpolation happens, they will read in from the file and create the lookup table themselves using their desired choice of log(x) vs. x, log(f) vs. f, and interpolant.  Perhaps the solution for now is to state in the dox that if a user provides a filename, the default interpolation is in P(k) rather than log(P) vs. log(k)?