v0.9.1

Hypatia v0.9.1

Diff since v0.9.0

Merged pull requests:

• remove extraneous Test (#856) (@araujoms)
• precise central point for relative entropy (#857) (@araujoms)
• avoid catastrophic cancellation in divided differences (#858) (@araujoms)
• precise central point for logarithm (#859) (@araujoms)
• precise central point for power mean (#860) (@araujoms)
• release 0.9.1 (#861) (@araujoms)