LinkML scheme should have min/max values as integers rather than string representation of integers

[ddooley]

linkml-validate code fails right now if schema has "1234" string numbers, rather than 1234 numbers directly, i.e. a DH schema with

minimum_value: "0"
maximum_value: "130"

whereas this succeeds:

minimum_value: 0
maximum_value: 130

So a small fix to DH tabular_to_schema.py achieves this.