Unit validations

[noracato]

Add unit validations for Inputs using a Unit model.

1. Change the unit field of the input from a string to a model
2. For now the unit will have only one string attribute that will be validated by a constant list of all supported unit types. We do not care for conversions for now.
3. The Unit should have a to_s or to_string method that will return the unit in human readable string format (check conventions in Python)