Add complex number support to atan

[kgryte]

This PR

• adds complex number support to atan by documenting special cases. By convention, the inverse tangent has a branch cut placed at the line segments (-infinity j, -j) and (j, infinity j) of the imaginary axis.
• updates the input and output array data types to be any floating-point data type, not just real-valued floating-point data types.
• derives special cases from C99

[rgommers]

LGTM, thanks @kgryte. This PR has been open for a week and the complex number basic principles (branch cuts, type promotion, etc.) have been reviewed in separate PRs already. So I'll merge this.