Pull request ExternalDecay plugins to add to 6_2_X

This pull request mirrors the changes made in 5_3_X, minus the TauSpinnerInterface because it 6_2_X is using Tauola++ 1.0.5 which does not have this functionality yet. Once these changes are in 6_2_X we can move to newer versions of the Tauola++ without breaking anything (ie Add TauSpinner).  I recommend the same procedure as 5_3_X were we built a special release for Relval and then merge into 6_2_X after the full relVal.