possibly deprecate Annuity.pt_bal()

[genedan]

See Vaaler and Daniel example 3.5.3. This method was originally written to solve for the value of a deferred annuity several years beyond the last payment.

Now that the Annuity class takes an argument for deferrals, pt_bal() returns the same answer as eq_val(), the equation of value for the underlying payments class. Because of this, I'm not so sure if pt_bal() needs to exist or repurposed for something else.

An alternative would be to rewrite the method to return the value of an annuity t years after the last payment.