Deepcopy fix

[dmuldrew]

Generalization of grid property access produced a bug:

>>> scenario.state.prepare_simulation_input()
---------------------------
PREPARING SIMULATION INPUTS
---------------------------
100%|################################################################################################################| 211/211 [00:00<00:00, 3.82kb/s]
--> Creating temporary folder on server for simulation inputs
--> Copying demand base profile into temporary folder
--> Copying hydro base profile into temporary folder
--> Copying solar base profile into temporary folder
--> Copying wind base profile into temporary folder
--> Preparing MPC file
Scaling grid
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/brdo/REM/PowerSimData/powersimdata/scenario/execute.py", line 106, in prepare_simulation_input
    si.prepare_mpc_file()
  File "/Users/brdo/REM/PowerSimData/powersimdata/scenario/execute.py", line 191, in prepare_mpc_file
    grid = self.scaler.get_grid()
  File "/Users/brdo/REM/PowerSimData/powersimdata/input/scaler.py", line 56, in get_grid
    self._grid = copy.deepcopy(self._original_grid)
  File "/Users/brdo/.pyenv/versions/3.6.5/lib/python3.6/copy.py", line 159, in deepcopy
    copier = getattr(x, "__deepcopy__", None)
  File "/Users/brdo/REM/PowerSimData/powersimdata/input/grid.py", line 65, in __getattr__
    return self.fields[field_name]
KeyError: '__deepcopy__'

The solution was to explicitly call these methods in the inherited object class.

[rouille]

All tests pass and test scenario was created and prepared successfully. Thanks for the quick fix.