Programatic interface for instance validation

[kenloen]

Ported from comment on #75

When a user wants to validate an instance of the windIO schema they are required to know the "path" or "name" of the schema (e.g. plant/wind_farm, turbine/turbine_schema) which can be a slight inconvenience that one might have to leave to script to figure out the name of the schema.

To make this interface slightly more accessible, we could consider changing the windIO.validate method into a nested class where the different schemas are attributes, something along the lines of:

class PlantValidator:
  def __call__(self, file_or_data):
    validate_yaml(file_or_data, <plant-schema.yaml>)

class TurbineValidator:
  def __call__(self, file_or_data):
    validate_yaml(file_or_data, <turbine-schema.yaml>)

class windIOValidator:
  plant = PlantValidator()
  turbine = TurbineValidator()

Where the validation would then look something like:

from windIO import windIOValidator

windIOValidator.plant(file_or_data")

windIOValidator.turbine(file_or_data)

Another option would also be to serve our schemas and then users would be able to add the $schema key to the instance in which case the validator would be able to figure out which schema to use. This is linked with this issue #31 .