Possibility to globally bias MeanVertex objects via env.var

[shahor02]

Following ITS realignment the mean vertex position may change. To avoid recalibration of the huge amount of MeanVertex objects in the CCDB, one can globally bias them via env.var O2_DPL_MVBIAS providing the --configKeyValues-like string for the MeanVertexBiasParam ConfigurableParam ("mvbias"). E.g.

export O2_DPL_MVBIAS="mvbias.xyz[0]=0.002;mvbias.xyz[1]=-0.12;mvbias.slopeY=0.0005"

will shift all loaded positions by 0.002 cm in X and -0.12 cm in y and increase the y slope by 0.5mrad.
At the very first call of MeanVertex constructor, the bias will be reported.

Example:

• w/o defining the O2_DPL_MVBIAS above:

root [0] auto& cc = o2::ccdb::BasicCCDBManager::instance();
root [1] cc.setTimestamp(1763178182869);
root [2] auto mv = cc.get<o2::dataformats::MeanVertexObject>("GLO/Calib/MeanVertex");
[INFO] Mean vertex is biased by: XYZ: 0.0000,0.0000,0.0000 SlopeXY: 0.000e+00,0.000e+00
[INFO] ccdb reads http://alice-ccdb.cern.ch/GLO/Calib/MeanVertex/1763178182869/50a76db2-c1d6-11f0-800c-0aa200331b9a for 1763178182869 (retrieve, agent_id: root.exe/root.exe@2031891 target/Linux_x86_64 init/1782229189459 id/shahoian@alicers05a session/mfgNGSYG), 
root [3] mv->print();
Vtx {-4.1312e-02,-5.5656e-03,+5.2010e-01} Cov.:{{4.419e-06..},{0.000e+00,4.326e-06..},{0.000e+00,0.000e+00,3.147e+01}} | bias: XYZ: 0.0000,0.0000,0.0000 SlopeXY: 0.000e+00,0.000e+00

• with O2_DPL_MVBIAS defined as above:

root [0] auto& cc = o2::ccdb::BasicCCDBManager::instance();
root [1] cc.setTimestamp(1763178182869);
root [2] auto mv = cc.get<o2::dataformats::MeanVertexObject>("GLO/Calib/MeanVertex");
[INFO] Mean vertex is biased by: XYZ: 0.0020,-0.1200,0.0000 SlopeXY: 0.000e+00,5.000e-04
[INFO] ccdb reads http://alice-ccdb.cern.ch/GLO/Calib/MeanVertex/1763178182869/50a76db2-c1d6-11f0-800c-0aa200331b9a for 1763178182869 (retrieve, agent_id: root.exe/root.exe@2033006 target/Linux_x86_64 init/1782229288457 id/shahoian@alicers05a session/UfLaIf6A), 
root [3] mv->print()
Vtx {-3.9312e-02,-1.2557e-01,+5.2010e-01} Cov.:{{4.419e-06..},{0.000e+00,4.326e-06..},{0.000e+00,0.000e+00,3.147e+01}} | bias: XYZ: 0.0020,-0.1200,0.0000 SlopeXY: 0.000e+00,5.000e-04