alternative to hard-coded ___indentation in environmental context value sets: is_a relations between permissible values

[turbomam]

@patrick has suggested asserting that the 'urban biome' permissible value in https://microbiomedata.github.io/submission-schema/EnvBroadScaleSoilEnum/ is_a 'dense settlement biome'. That could lead to a presentation with underscore indentation, or some other presentation, or other presentations in different systems