rename AntiMinus to PrePlus
Merge pull request #109 from sn6uv/PrePlus

rename AntiMinus to PrePlus
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
refactor _MPMathFunction to work with multiple arguments
MPMathFunctions should use sympy on symbolic arguments not just exact numbers
implement LegendreP
implement JacobiP
implement SphericalHarmonicY
implement GegenbauerC