psi: add support to compute the polygamma function (kth-derivative). * libinterp/corefcn/psi.cc: previously, only the digamma function, k == 0, was being computed. Add support for polygamma function, add tests, and improve documentation. * liboctave/cruft/slatec-fn/dpsifn.f, liboctave/cruft/slatec-fn/psifn.f: the two functions that actually compute the the polygamma functions, copied verbatim from SLATEC, and under public domain. * liboctave/cruft/slatec-fn/module.mk: add dpsifn.f and psifn.f to the build system. * liboctave/numeric/lo-specfun.cc: add new signature for function psi to compute polygamma function that wraps the Fortran DPSIFN and PSIFN functions. * liboctave/numeric/lo-specfun.h: declare new function and document all psi() with doxygen.

--- a/liboctave/cruft/slatec-fn/module.mk	Sun May 03 03:16:13 2015 +0100
+++ b/liboctave/cruft/slatec-fn/module.mk	Sun May 03 22:52:07 2015 +0100
@@ -34,6 +34,7 @@
   cruft/slatec-fn/dlnrel.f \
   cruft/slatec-fn/dpchim.f \
   cruft/slatec-fn/dpchst.f \
+  cruft/slatec-fn/dpsifn.f \
   cruft/slatec-fn/erf.f \
   cruft/slatec-fn/gami.f \
   cruft/slatec-fn/gamit.f \
@@ -44,6 +45,7 @@
   cruft/slatec-fn/inits.f \
   cruft/slatec-fn/pchim.f \
   cruft/slatec-fn/pchst.f \
+  cruft/slatec-fn/psifn.f \
   cruft/slatec-fn/r9lgmc.f \
   cruft/slatec-fn/r9lgit.f \
   cruft/slatec-fn/r9gmit.f \