cleanup of _MPMathFunction by adding mpmath_name analagous to sympy_name
implement Zeta
implement numeric BesselJ. Fixes #100
implement numeric BesselJ
cleanup Bessel type functions and add BesselI and BesselK
implement HankelH1 and HankelH2
add numerical Bessel related functions: Airy, Kelvin, Struve, more Bessel and Airy functions
add tests to Bessel related functions
add Bessel related function docs
cleanup Bessel function docs and add plot examples
correct Bessel function attributes
refactor AiryAiZero and AiryBiZero