Add Metric-122 20 as an unofficial round

[jatkinson1000]

@mjtamlyn requested the addition of a Metric-122 20 round for use in the Wallingford Castle club shoots.

This should be fairly straightforward. Couple of points to consider however:

• It will be easiest to add this as a custom round
• However, it might be useful to incorporate it into some other functionalities (classifications by event?) and group in the same family as other Metric-122 rounds
• But we need to be sure it doesn't appear in classification tables and is flagged as unofficial for HC/classification purposes
• Are there any other rounds that could also be added at the same time? - Metric 122-80 20?

To make these changes we need to:

• Add the round in the archeryutils package
• Add any necessary functionality in archerycalculator to blacklist from certain operations etc.