Fix geometry overlap by clipping the upstream face of the DetectorArray mother volume using a subtraction solid

[zdemirog]

This PR fixes the overlap between GEM detector mother volume and Thin quartz mother volume.

After GEM detector implementation (PR625), the following overlap warnings started to appear in the console outputs;

G4GDML: Reading 'mollerMother.gdml' done!
Stripping off GDML names of materials, solids and volumes ...


-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : GeomVol1002
      issued by : G4PVPlacement::CheckOverlaps()
Overlap with volume already placed !
          Overlap is detected for volume GEM_world_log_PV:0 (G4Polycone) with DetectorArray_volume_PV:0 (G4Cons)
          overlap at local point (897.968,699.826,-1228.1) by 2.19 cm  (max of 118 cases)
NOTE: Reached maximum fixed number -1- of overlaps reports for this volume !
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------


-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : GeomVol1002
      issued by : G4PVPlacement::CheckOverlaps()
Overlap with volume already placed !
          Overlap is detected for volume DetectorArray_volume_PV:0 (G4Cons) with GEM_world_log_PV:0 (G4Polycone)
          overlap at local point (-674.779,847.342,1445.1) by 2.19 cm  (max of 39 cases)
NOTE: Reached maximum fixed number -1- of overlaps reports for this volume !
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------

To solve this, the upstream face of the DetectorArray mother volume is now clipped using a subtraction solid, removing the region that overlaps with GEM mother volume. Please see the screenshot.

[zdemirog]

@rakithab, I am sure I didn't change tile positioning, but I would like to verify it. So I'll run simulations to confirm that. I'll update this PR once I have the results.

[zdemirog]

@rakithab, I am sure I didn't change tile positioning, but I would like to verify it. So I'll run simulations to confirm that. I'll update this PR once I have the results.

The simulation results show that tile positions are identical.