creates a default set of Opal objects called 'default.opals
By default if there is only one set of opals that is available for analysis, all DataSHIELD client-side functions will use that full set of Opals unless the 'datasources=' argument has been set and specifies that a particular subset of those Opals should be used instead. The correct identification of the full single set of opals is based on the datashield.connections_find() function which is an internal DataSHIELD function that is run at the start of nearly every client side function. To illustrate, if the single set of Opals is called 'study.opals' and consists of six opals numbered study.opals to study.opals then all client-side functions will use data from all six of these 'study.opals' unless, say, datasources=study.opals[c(2,5)] is declared and only data from the second and fifth studies will then be used. On the other hand, if there is more than one set of Opals in the analytic environment client-side functions will be unable to determine which set to use. The function datashield.connections_find() has therefore been written so that if one of the Opal sets is called 'default.opals' then that set - i.e. 'default.opals' - will be selected by default by all DataSHIELD client-side functions. If there is more than one set of Opals in the analytic environment and NONE of these is called 'default.opals', the function ds.setDefaultOpals() therefore copies one set of opals and to name that copy 'default.opals'. This set will then be selected by default by all client-side functions, unless it is deleted and an alternative set of opals is copied and named 'default.opals'. Regardless how many sets of opals exist and regardless whether any of them may be called 'default.opals', the 'datasources=' argument overrides the defaults and allows the user to base his/her analysis on any set of opals and any subset of those opals. An earlier version of 'datashield.connections_find()' asked the user to specify which Opal to choose if no default could be identified, but that did not work in all versions of R and so has been removed.
Copies a specified set of Opals (on the client-side server) and calls the copy 'default.opals'
Burton, PR. 28/9/16