1.7.23 Refactor `plot_ordination` to be more stable, error less and give informative warnings

CHANGES IN VERSION 1.7.23
-------------------------

USER-VISIBLE CHANGES

	- Refactor `plot_ordination` to be more stable, error less and give
informative warnings.

	- Expect no critical API changes. Some errors now informative warnings
with useful auto-changes to parameters.

	- The `type='biplot'` option no longer hard-specifies a discrete color
scale. Available default pallette should work.

	- For `type='biplot'`, the non-variable (Taxa or Sample) label will
always appear first in a discrete legend.