simple subfields

it seems to me there is support for repeating subfields but not subfields that are not repeating. I find it very convenient to organize the schema to support subfields.

example:
```
{
"field_name": "vertical-extent",
"label": "Vertical Extent",
"simple_subfields": [{
"field_name": "min",
"label": "Min",
"form_attrs":{"pattern":"[0-9.]+"}
},
{
"field_name": "max",
"label": "Max",
"form_attrs":{"pattern":"[0-9.]+"}
}
]
}
```

While this could be done with two fields it is more obvious to users that the min and max values are related when grouped as subfields.

Start by locating the existing implementation and schema examples for repeating subfields, since no files or tests are named in the issue. Add support for non-repeating simple_subfields such as the shown min/max grouping, including their form attributes, and verify that related values remain grouped and usable in the schema.

Written by the indexing model from the issue text.