Readjust whitespace to original conventions.
Use documented include statements for Eigen
Don't #include <iostream>.  It's not needed.
variable(): Avoid memory overhead of identity constructor.
Remove two instances of EOL whitespace.
Implement constant() and variable() in terms of function()

Specifically,

  - A constant is a function with zero derivative
  - A variable is a function with unit derivative with respect to
    itself.
Completed basic version of AD vector class and test.
Initial work in progress on block AD class.