Initial renovation of arithmetic.py docs.
Documentation improvements for arithmetic.py

Added or expanded the documentation for a few functions, and improved
the consistency of the formatting used.
Minor documentation edit.
Merge pull request #119 from freshwater/Docs/arithmetic.py

Documentation improvements for arithmetic.py
Merge pull request #80 from sn6uv/Import

Import
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