make_mean_pephylo: Write PEPHYLO input mean traits
In rscherrer/bop:

This function saves the MetricTraitMeans.txt input file for PEPHYLO.

1	make_mean_pephylo(data, outpath, nPC, whatSex)

`data`	A data frame or matrix with dependent variables in columns and observations in rows.
`outpath`	A string. The path to the folder where the output file is to be saved.
`nPC`	How many PC to retain? Either an integer i, then PC 1 to i will be retained, or a vector of integers representing what PCs to retain.
`whatSex`	A logical vector of length the number of observations in the principal component, with `TRUE` for each observation of the right sex.