SAMMON: set _is_initialized as true after init()

[zingarelli]

In Sammon's Mapping, property _is_initialized is not being set to true in the init() method. This property is false by default.

When transform() or generator() methods are called and return this.projection, the get projection() from class DR will check _is_initialized and, as it's false, will call the init() method again, thus replacing the values computed for the projection with the initial values.

Setting _is_initialized as true in the end of the init() method fix this problem.