Release of JDecisiontableLib 2.1.0

The main reason to release JDecisiontableLib 2.1.0 is that I missed read-only interfaces for the mutable classes Decisiontable, Node and Rule (all in package de.mgmechanics.jdecisiontablelib.dt).

These interfaces, named IRo.* ("Interface for Read-Only access") provide access to all methods which do not alter the state of an object. So these interfaces provide a view to these objects. Using these interfaces you render instances of these classes immutable.

In opposite to the interfaces made for serialisation, named ISer.* "Interface for Serialisation", which provide access to only those methods which are do not alter the state of an object AND are necessary to create new instances of the implementing class.