mergeMaps documentation does not describe merging algorithm

[drserajames]

The documentation for mergeMaps does not describe how the titres are merged (but it is very well described in the code, which I reproduce below).

I can update the documentation at some point.

1. If there are > and < titers, result is "*"
   2a. If there are just ".", result is "."
2. If there are just "" or ".", result is ""
3. If there are just lessthan titers, result is min of them, keeping lessthan
4. If there are just morethan titers, result is max of them, keeping morethan
5. Convert > and < titers to their next values, i.e. <40 to 20, >10240 to 20480, etc. and take the log
6. Compute SD, if SD > options.sd_limit, result is *
7. Otherwise return the mean (ignoring nas)