Alternative interface for Coordinate class

Hi there!

Follows an alternative interface to the Coordinate class. I think CamelCase doesn't mix well with underscores, so I would prefer `getCoordX()` over `getCoord_x()`, or simple `x()` -- but of course this is a matter of taste.

Usually a simpler interface is the best to choice for use and maintanance. So `setCoord_x()` and `setCoord_y()` could become a single method. If someone is willing to change only a single coordinate them `c.setCoord(c.x(), 7)` could be used.

A `toString()` method could also be handy for tests:
* create a empty board
* create a piece
* move the piece
* check that `piece.getCoord().toString()` matches the expected value.