QIIME 2 validator: warn if cells contain leading/trailing whitespace

[jairideout]

Improvement Description It'd be nice to have a few different warning cases:

• If the cell is whitespace-only, note that the cell will be treated as an empty cell in the message.
• If the cell has content with leading/trailing whitespace, note that the leading/trailing whitespace will be ignored.
• If the cell is whitespace-only and is part of a trailing empty column, note that it contains whitespace and that the cell will be ignored altogether because it is part of a trailing empty column.