Allow passing instructions to @model

[jlowin]

You can pass instructions to steer model transformation via the instructions parameter:

@marvin.model(instructions='Always generate locations in California')
class Location(BaseModel):
    city: str
    state: str

Location('a large city')   
# Location(city='Los Angeles', state='California')

Note that instructions are set at the class level, so they will apply to all instances of the model. To customize instructions on a per-instance basis, use cast with the instructions parameter instead.