Resolve "Add options for envelopes where we use assemblies"

For disks of homogenous modules, this adds an envelope flag to place these modules in an envelope volume instead of an assembly. This makes the geometry tree more balanced, may reduce lookups, factorizes overlap checks for speedups, and possibly speeds up jsroot display. However, it prevents the use of very closely stacked detector systems that would overlap in their envelope volumes.

Usage:

    <detector id="2" name="DiskShapeCalorimeter" type="HomogeneousCalorimeter" readout="EcalHits">
      <position x="0" y="0" z="-30*cm"/>
      <rotation x="0" y="0" z="0"/>
      <placements>
        <disk rmin="25*cm" rmax="125*cm" length="20.5*cm" phimin="0" phimax="360*degree"
              envelope="false" sector="1">
          <module sizex="2.05*cm" sizey="2.05*cm" sizez="20*cm" vis="GreenVis" material="PbWO4"/>
          <wrapper thickness="0.015*cm" material="Epoxy" vis="WhiteVis"/>
        </disk>
      </placements>
    </detector>

There is support for envelope="true,[false]" and material="[Air]".

compact/ecal_backward_PbWO4.xml

@@ -25,7 +25,7 @@
         <disk
             rmin="EcalEndcapN_rmin"
             rmax="EcalEndcapN_rmax"
-            length="EcalEndcapN_length"
+            envelope="true"
             sector="1">
           <module
             sizex="EcalEndcapN_CrystalModule_width"