Fix newly added enum.py statement to eliminate FutureWarning of deprecation

[martinholmer]

@nikhilwoodruff, a few days ago in commit 7598d96 you added this statement:

policyengine-core/policyengine_core/enums/enum.py

Lines 56 to 57 in 92e3bb6

	if isinstance(array[0], Enum):
	array = np.array([item.name for item in array])

This statement generates a dozen FutureWarnings of a deprecation as described here:

It would be good to fix this before it becomes an error.

[MaxGhenis]

Just involves replacing array[0] with array.iloc[0]?

[martinholmer]

@MaxGhenis said in issue #238:

Just involves replacing array[0] with array.iloc[0]

Yes, I think so. Certainly that shouldn't be too burdensome for @nikhilwoodruff