Add complex number support to sign

[kgryte]

This PR

• adds complex number support to sign.
• defines the sign to be x/|x| ensuring a consistent definition for real and complex data types (as discussed in Computing the sign of a complex number #545).
• closes Computing the sign of a complex number #545.
• requires that special cases follow the rules of complex number division. In this case, the numerator is a complex number and the denominator a real value and, thus, complex number division is well-defined.
• updates the input array data type to be any numeric data type, not just real-valued data types.
• adds a special case for real-valued inputs in that NaN should be returned if x_i is NaN.
• depends on Add support for complex number division #554.

[rgommers]

This looks consistent with what was discussed in the linked issue. LGTM, in it goes. Thanks @kgryte