LifeTable: Closing methods

[mpascariu]

The actual version of the package uses a simple method to close a life table. It works well in the most common cases but when dealing with abridged life-tables and various indices more particular methods and flexibility is required.

In order to come up with a solution, we have to figure out:

1. What are the best practices?
2. Which methods are most reliable in most of the situations?

This issue is related to issue #7 .