Make some TableModel methods final?

[nishihatapalmer]

Overriding the tablemodel methods causes TreeTableModel to fail in interesting ways unless great care is taken.

Some users may do this because they are more familiar with TableModel, but TreeTableModel is implementing everything for you.

I kind of don't like restricting how people use this, there may be valid use cases for overriding some of these methods sometimes.

Possibly just document this as a risky area and give examples on how to do it safely.

[nishihatapalmer]

Added documentation to be careful overriding methods. Don't really want to restrict this technically.