rename `pcoa` parameter name?

[sjanssen2]

We have introduced t-SNE and UMAP as alternative ordination methods in qiime2-2021.11. Should we therefore change the input parameter name for the ordination (which previously always was a PCoA) from "pcoa" into "ordination"?

q2-emperor/q2_emperor/plugin_setup.py

Line 65 in fdd1a8b

'pcoa': 'The principal coordinates matrix to be plotted.'

[ElDeveloper]

I agree with this! Maybe the way to do this is to first create a new type that's named "OrdinationResults" not "PCoAResults". Then we can add that type to the relevant actions and rename where appropriate. @thermokarst @ebolyen any suggestions on how to do this with backwards compatibility in mind?