group: add option to ignore missing samples

[nbokulich]

Bug Description
group raises an error if the metadata-column has missing values. There should be an option for handling missing samples, e.g., error or drop.

This behavior is contradictory to the way QIIME 2 is supposed to handle metadata: i.e., missing values are preferred to "NA" strings.

Steps to reproduce the behavior
Group a feature table using a metadata column containing empty values

Expected behavior
There should be options to a) ignore (drop) or b) aggregate all samples with missing values into a single group.

Computation Environment

• mac OS 10.11.6
• QIIME 2 Release 2018.11