Nested/Dictionary data values

Nobody has claimed this yet.

Hello,

It seems like it is possible to create a `Record` with a dictionary as its data value, but not possible to insert that `Record` into a `DataStore`. Here is a minimal example

```python
from sina.model import Record
from sina.datastore import create_datastore

rec = Record('a', 'A')
rec.data_values['field'] = {'b': 5, 'c': 6}

ds = create_datastore('test.sql')
ds.records.insert(rec)
```

returns

`ValueError: ['At least one data entry belonging to Record a has a dictionary for a value.Value: field']`

I figured this would not be allowed as each data can also have a `units` field, but I have heard that there is some sort of option to use nested data. If there is a way to do so I would love to know more. Thanks.

Start with the Record data_values assignment, create_datastore('test.sql'), and ds.records.insert(rec) path shown in the reproducer. Trace where dictionary values are rejected and determine whether nested values should be supported or documented as invalid; done means the intended behavior is consistent for Record creation and DataStore insertion.

Written by the indexing model from the issue text.