grouping update

• introduction of the function setGroups(), which allows to summarize columns and/or rows according to a function. The positions and summarise function here are provided via the new function .sum().
• now each getter ensures that it only gets things from an already validated schema.
• for filter and group slots of a schema there is now a new way implemented for storing locations. Each location statement is now a list that contains either find, group or position parameters.