simplemap should support link mapping

[karsany]

new function should be added VAU's simplemap grammar and parser.
should handle link mappings too.

map for entity EMPLOYEE {
    source {
        [...]
    }
    business key EMPLOYEE_ID;
    attributes {
        [...]
    }
    #### new from here ####
    link with JOB through JOB_ID { 
        datagroup [...]
    }
    #### new to here ####
}