Removed abstract doubleValue() method from NodeConstant

Method moved to NodeNumber class which can be accessed by using a
NodeTransformer on each NodeConstant.

This commit contains multiple build errors as a result
Refactorings to replace all doubleValue() calls

As the doubleValue() method was removed from the NodeConstant class
all references have been replaced with a call to a NodeTransformer in order
to first get a NodeNumber representation and then get its double value.