Add Q3eR field rotation (#837)

### Briefly, what does this PR introduce?
The geometry of the Q3eR magnet was rotated but the field volume wasn't
rotated to match.

### What kind of change does this PR introduce?
- [x] Bug fix (issue #__)
- [ ] New feature (issue #__)
- [ ] Documentation update
- [ ] Other: __

### Please check if this PR fulfills the following:
- [ ] Tests for the changes have been added
- [ ] Documentation has been added / updated
- [ ] Changes have been communicated to collaborators

### Does this PR introduce breaking changes? What changes might users
need to make to their code?
No
### Does this PR change default behavior?
No

Author: simonge

compact/far_backward/magnets.xml

@@ -78,7 +78,7 @@
     type="CylindricalMagnetChain"
     vis="RedVis">
       <magnet id="0" name="Magnet_Q3eR"
-        x="(Q3eR_StartX+Q3eR_EndX)/2" y="0" z="(Q3eR_StartZ+Q3eR_EndZ)/2" theta="Q3eR_Theta"
+        x="Q3eR_XPosition" y="0" z="Q3eR_CenterPosition" theta="Q3eR_Theta"
         length="Q3eR_Length" rin="Q3eR_InnerRadius" rout="4*Q3eR_InnerRadius">
       </magnet>
     </detector>
@@ -105,7 +105,8 @@
 
     <field name="Magnet_Q3eR_Field" type="MultipoleMagnet">
       <shape type="Tube" rmax="Q3eR_InnerRadius" dz="Q3eR_Length/2"/>
-      <position x="0" y="0" z="Q3eR_CenterPosition"/>
+      <position x="Q3eR_XPosition" y="0" z="Q3eR_CenterPosition"/>
+      <rotation x="0" y="Q3eR_Theta" z="0"/>
       <coefficient/>
       <coefficient coefficient="Q3eR_Gradient"/>
     </field>