clarify how zeros are dealt with for the geomeans

[eeholmes]

The documentation is unclear.

from email with Mari: "Ok well forget everything I wrote before. For the raw geomeans, 0s are replaced with NA, not quite on purpose but because all the code works on the log-scale and log(0) = NA. I say not quite on purpose because in the modeling all work is done on the log-scale, so 0s do become NAs. I'd forgotten that when computing the raw geomeans that I also work with the log data so 0s become NAs. Well, again add that to the things to revisit for next go round."