optimized mapping - datatypes

we can retrieve datatypes from bidsschematool (not now but for revising the converter later)
```
import bidsschematools as bst
import bidsschematools.schema

schema = bst.schema.load_schema()
# list of data types (high level folder structure in bids)
dtypes = [term for term in schema.objects.datatypes.values()]
for dt in dtypes:
    print(dt['value'])
print()
```