add error message for function.Array.cast of numpy object array

[joostvanzwieten]

The following example

from nutils.function import Array
import numpy
Array.cast(numpy.array(['a', 'b'], dtype=object))

gives the following error message

Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
    Array.cast(numpy.array(['a', 'b'], dtype=object))
  File "<stdin>", line 144, in cast
    value = _Constant(value)
  File "<stdin>", line 752, in __init__
    self._value = types.arraydata(value)
  File "<stdin>", line 573, in __call__
    return args[0].__new__(*args, **kwargs)
  File "<stdin>", line 747, in __new__
    dtype = dict(b=bool, u=int, i=int, f=float, c=complex)[array.dtype.kind]
KeyError: 'O'

This is not really helpful. Consider adding a proper error message informing the caller that object arrays are not supported.