Make result_dtype in _create_method functional

[Julian-Harbeck]

Pandas requieres the result_dtype argument for the _create_method function of the UnitsExtensionArray class. This was added in 8c1f8b2, but it is currently not further used.

Clarify the following questions:

• Does result_dtype takes precedence over coerce_to_dtype argument?
• What if result_dtype is incompatible to the operator's return type?

Afterwards add implementation.