Allow sharing of source tables between different data sets

re-use of existing (already loaded) data tables can overcome some of the limitations we currently encounter: a. the fact that we can only load 1 reference model per data set. creating a 2nd data set based on the same loaded data tables will reduce the GBs but more important avoid (sometimes cumbersome/lengthy) reload of data. b. during development/enhancement of transformations realistic data can be used, without disturbing the 'live' enviroment