Unwrap function

[ChrisRackauckas]

After #7 we can do @unwrap_function on a function definition is:

@unwrap_function (u,nothing) function f(t,u)
  # code
end

gives

function f(t,u)
  @unwrap u
  # code
  @wrap u
end

so that way using the values by their names as variables "just works" in the function. (u,nothing) means to unwrap u without changing the variable names. (du,:d_) would mean to unwrap like d_a, d_b, d_c, etc. Example:

@unwrap_function (u,nothing) (du,:d_) function f(t,u,du)
  d_a = a*c
  d_b = b*c
  d_c = a*b + c
end

would be the same as

@unwrap_function (u,nothing) (du,:d_) function f(t,u,du)
  du[1] = u[1]*u[3]
  du[2] = u[2]*u[3]
  du[3] = u[1]*u[2] + u[3]
end

which is clearly a nicer way to write the diffeq.