This class is the basic implementation of the Level interface. It proviedes
also a static method 'evaluate' which permits to obtain the integer value
of a level expression.
author: Sebastien Chassande-Barrioz
It analyzes a string expression to obtain its integer value. The allowed
expression type are the following:
an integer value
another level name
levelName + integerValue
levelName - integerValue
Parameters: expr - is the string expression which must be evaluated. Parameters: lf - is the LevelFactory which permits to obtain the referenced level. an integer value or 0 if it is impossible to evaluate theexpression.