v0.9.1

NiLang v0.9.1

Diff since v0.9.0

Merged pull requests:

• add example for ChainRules rrule (#64) (@johnnychen94)
• use classic docs badge (#65) (@johnnychen94)
• add ChainRules to docs (#66) (@johnnychen94)
• format: link docstring to i_cor_cov (#69) (@johnnychen94)
• faster ad rule for power, more tests (#71) (@GiggleLiu)
• fix complex mul performance, bennett switch to _zero (#72) (@GiggleLiu)
• Fix complexmul performance (#73) (@GiggleLiu)