Add test for difference between end vertices of two composites

DaVinciExamples/python/DaVinciExamples/tupling/example-tupling-AllFunctors.py

@@ -70,6 +70,8 @@ def all_variables(pvs, DTFR, ptype):
         vars['VY'] = F.VY
         vars['VZ'] = F.VZ
         vars['VRho'] = F.VRho
+        vars['Ds_VZ'] = F.CHILD(1, F.VZ)
+        vars['Delta_VZ_DsB0'] = F.CHILD(1, F.VZ) - F.VZ
 
     #    vars['DOCA'] = F.DOCA(Child1=1,Child2=2)    # no member named 'doca' in 'LHCb::Particle' (?)
     #    vars['DOCACHI2'] = F.DOCACHI2(Child1=1,Child2=2)  # same here

DaVinciExamples/tests/qmtest/tupling.qms/test_davinci_tupling_All.qmt

@@ -69,6 +69,8 @@ B_vars_stored = ['B0_BPVCORRM'
 , 'B0_VX'
 , 'B0_VY'
 , 'B0_VZ'
+, 'B0_Ds_VZ'
+, 'B0_Delta_VZ_DsB0'
 , 'Kaon_BPVIPCHI2'
 , 'Kaon_CHARGE'
 , 'Kaon_DTF_BPVIPCHI2'