Bring back support of the overlap parameter in item-generation scripts

The overlap parameter has been removed from the .yml files and is no longer supported. It has been replaced by the shift parameter, which allows shifting each input file by an absolute value in seconds. However, this approach is impractical when designing IVAS characterization tests, where a specific overlap between items is required. The reason is that one must pre-calculate the shift value to achieve the desired overlap or gap.