psi: add support for complex numbers. * libinterp/corefcn/psi.cc: add logic and input check to support complex numbers (implementation is in lo-specfun.cc). Add tests. * liboctave/numeric/lo-specfun.cc, liboctave/numeric/lo-specfun.h: add template specialization to psi() for std::complex. It is mostly taken from the implementation in XLiFE++ (also under GPLv3+, see b03c7cccadc2 commit message for more details).

--- a/liboctave/numeric/lo-specfun.h
+++ b/liboctave/numeric/lo-specfun.h
@@ -665,5 +665,7 @@
 
 template<class T>
 extern OCTAVE_API T psi (const T& z);
+template<class T>
+extern OCTAVE_API std::complex<T> psi (const std::complex<T>& z);
 
 #endif