Bug in bmi_Ku.py when setting values

[ethan-pierce]

In the BMI for Ku, set_value() calls the built-in getattr(), which raises an error because getattr() can't assign values. This is also a problem with set_value_at_indices().

https://github.com/permamodel/permamodel/blame/3d091411d115a2617d4cb686e5c65b4c0cd9b926/permamodel/components/bmi_Ku.py#L231

[ethan-pierce]

Fixing this is not as simple as replacing getattr() with setattr(), as Ku expects xarray DataArrays for attribute fields related to model variables. (And the rest of the BMI expects to access field arrays using an attribute.values[:] syntax, which is not supported by numpy.)