Implemention-specific seed additions

[jalvord1]

We are going to run into circumstances where we need to add to the descriptor seeds but it might not make sense to add to the bundles because they are implementation-specific. For example, I'm seeing a GSN district with different PL values for a school benchmark PL in their star files. According to the star documentation, this field can be customized (but we've seen that most districts tend to use the default values). We'll need to consider how to handle these situations. Note: I recognize this could involve updates to EM, but storing as an issue here.