synchrony, Gross, with only 1 spp in a plot

[laurenmh]

If there is only one species, synchrony (Gross) returns the following error:
Error in cor(agg.otherspp, com.focalspp) : incompatible dimensions

Would be nice to add a check to make a clearer error, and also to allow the metric to run on other replicates with more species

[laurenmh]

Note this is also an issue for the variance ratio. Here the error is:
Error in cov(comdat, use = "pairwise.complete.obs") :
supply both 'x' and 'y' or a matrix-like 'x'

[laurenmh]

added a function to check this, returns the error:
Error in check_multispp(df, species.var, replicate.var) :
One or more replicates consists of only a single species;
please remove these replicates prior to calculations