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besselk does not implement derivative (and bessely/i not handle z=0 case for derivatives) #734

Description

@electroflow

Thank you for your very useful software package!

for funcname in ["besseli", "besselj", "besselk", "bessely"]:
    print(funcname, getattr(mpmath, funcname)(1, 1, derivative = 0),  getattr(mpmath, funcname)(1, 1, derivative = 1))

outputs

besseli 0.565159103992485 0.700906773759523
besselj 0.440050585744934 0.325147100813033
besselk 0.601907230197235 0.601907230197235
bessely -0.781212821300289 0.869469785515966

No warning or error message is raised that the derivative is not implemented for besselk. I think at least an error should be raised that the derivative is not implemented, this silent defaulting to derivative = 0 cost me a significant amount of debugging time.

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