Consider renaming realpart to value or primalpart or similar

This package works a treat for evaluating the derivative functions at complex locations:

julia> using DualNumbers

julia> f(x) = (1, -2 *x) .* exp(-x^2) # value and derivative
f (generic function with 1 method)

julia> z = 1.0 + im # some complex location to evaluate the derivative
1.0 + 1.0im

julia> f(z)
(-0.4161468365471424 - 0.9092974268256817im, -0.9863011805570786 + 2.650888526745648im)

julia> realpart(f(Dual(z, 1))[1])
-0.4161468365471424 - 0.9092974268256817im

julia> dualpart(f(Dual(z, 1))[1])
-0.9863011805570786 + 2.650888526745648im

In this instance, realpart returns a complex value, which is a little misleading. @goretkin suggested in Slack that primalpart could be a better adjective. Any thoughts?

Start by reviewing the package's realpart and dualpart APIs and searching for their uses in the repository. Compare the naming alternatives raised in the issue, then check the example to determine which name best describes the returned value. Done means the project has a decided, consistently applied name with working examples and tests.