Implement symbolic confluent hypergeometric functions

[eviatarbach]

This patch implements the Kummer confluent hypergeometric functions hypergeometric_M and hypergeometric_U (superseding the previous numerical-only version), complete with numeric evaluation, simplification, and transformation to generalized hypergeometric functions.

Depends on #2516
Depends on #16697

CC: @kcrisman @benjaminfjones

Component: symbolics

Keywords: hypergeometric, kummer

Author: Eviatar Bach, Ralf Stephan

Branch/Commit: 23a5c65

Reviewer: Jan Keitel, Ralf Stephan, Nils Bruin, Paul Masson

Issue created by migration from https://trac.sagemath.org/ticket/14896

[eviatarbach]

comment:1

Attachment: trac_14896.patch.gz

Whittaker functions can be easily implemented similarly; the reason I didn't include them in this patch is because the Maxima conversion is a bit more complicated.

[eviatarbach]

Attachment: trac_14896_2.patch.gz

[eviatarbach]

comment:2

Last patch, to be applied independently, adds LaTeX names which I forgot in the original.

[fchapoton]

Changed keywords from none to hypergeometric