Cardinality for ENVO terms should be 1 for now

[ramonawalls]

In reviewing mixs_table_stage modifications, I see that the cardinality ("occurence") of three ENVO-based terms (env_medium, env_local_scale, env_broad_scale) is set to "1", but there is a comment that such a choice may be controversial. However, when I look at older versions of MIxS, I see that occurence was set to 1 for those terms, so I think we should keep it as is until someone requests otherwise. This means no change is needed to your file, @turbomam.