describe to report if content in contributed_datapoint fails to conform

[HedvigS]

It would be great if the check segment of pygrambank describe checked if all the content in the column [C|c]ontributed_[D|d]atapoints" matched to a known coder abbreviation and if there is more than one person listed, check that they are separated by spaces and not commas or semicolons.

I found some misaligned comments sometimes in this col and inconsitent separation when more than one, would be neat to just check that while we're checking everything else for new submissions.

[xrotwang]

Individual coders are determined by looking for chunks of uppercase letters - regardless of separator: https://github.com/grambank/pygrambank/blob/f3e82e10d5fc39c5c64737dbafc158d7c11af905/src/pygrambank/sheet.py#L29-L31

The check against known coders is done in grambank describe: https://github.com/grambank/pygrambank/blob/f3e82e10d5fc39c5c64737dbafc158d7c11af905/src/pygrambank/commands/describe.py#L94-L98