御手 洗潔

Updated differentiate.jl to be make finite differences based on a symbolic expression

2

Does doing this make more sense than putting it into finite_difference and having it called by derivative()? ``` Julia julia> differentiate(:(e^x),:(x)) :(*(^(e,x),log(e))) julia> using Calculus julia> differentiate(:(e^x),:(x),1.0) 2.718281828459045 julia> differentiate(:(e^x),:(x),0.0)...