change farmer_pk and pool_pk to type str

[cciollaro]

currently when supplying these values you get the following exception:

marshmallow.exceptions.ValidationError: {'plotting': {'farmer_pk': ['Not a valid integer.'], 'pool_pk': ['Not a valid integer.']}}

[altendky]

I feel like these should maybe be something more specific so they can't be strings like 'the blue & green 37s are *$*#&', but str is still better than int. Thanks for the fix @cciollaro.