FFG Piercer not recognized

[leppott]

Describe the bug
There are two invertebrates in MD with the FFG characteristic "Piercer". If this designation is included in the dataset, it fails to run. Please add this to the valid inputs list.

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Expected behavior
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Additional context
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[leppott]

Modified metric.values() function to allow for Piercer.

• Added to details. (line 39)
• Added to QC check. (line 230)
• Added to FFG column creation (line 272)