Allow ExperimentType to be set

Remi Mommsen discovered that there is no way to set the ExperimentType data member in ProducerSourceBase, which is the base class of FRDStreamSource.
This simple pull request fixes this problem in 7_3_X.
Remi requested the problem be fixed in 7_2_X and 7_3_X.
A separate pull request will be used for 7_2_X.