Add option to load SS_output from RData file

[iagomosqueira]

For long grids a possible option to speed up loading is to avoid the multiple calls to r4ss::SS_output by creating and RData, or RDS, file with the result of a call to SS_output, either during the call to ss3 in conditioning, or on a subsequent foreach loop.

readss3() functions could accept as location the RData file and load the out list from there, rather than having repeated calls to SS_output by each readss3 function.