Add unit tests for the new life_expect functions (life_expect_mean and life_expect_var)

The new functions from Rob are life_expect_mean and life_expect_var.
These replace the old function life_expect.

We need unit tests for the new functions.

Note: I have kept the old function, life_expect in the package but have added a .Deprecated note to it.
I want to keep it working for selfish reasons (I have a bunch of code I am working on that relies on that function!).