1.9.11 robust plot_ordination w/ support for wt avg coord

inspired by
https://github.com/joey711/phyloseq/pull/364

CHANGES IN VERSION 1.9.11
-------------------------

USER-VISIBLE CHANGES

	- More robust `plot_ordination` behavior with clearer warning/error messages.

	- Coordinates automatically checked/assigned to OTU or samples

	- Attempt to calculate OTU or sample weighted-average coordinates via `vegan::wascores`, if-needed

	- Species weighted-average via `vegan::wascores` supported now in phyloseq::scores.pcoa