"all" is a somewhat confusing name for the default strata

When there is only one strata which hasn't explicitly been defined, epymorph uses the default value "all". This can lead to parameters which reference "gpm:all" in the name in a user-visible way. However I'm now worried that this name might be misleading and people confuse its function with "*".

Recommend changing the default strata name to something like "default".