Beamline magnet placement and geometry (#27)

I finished working on geometry of the Roman pots, so now I have some time to work on related elements.

@whit, from what I can tell, there are three types of magnets: octupoles, quadrupoles and the spectrometer magnet. I know the dimensions of the quadrupole and spectrometer magnet, but I'm missing info on the octupole. Do you have any fresher materials than the March IR Design status presentation?

src/GenericDetectors/beamline/compact/eic/B0pF_example.xml

+<detectors>
+  <detector id="3" name="SpectrometerMagnet" vis="RedVis" type="B0pFMag">
+    <placement x="0*m" y="0*m" z="0*m" theta="0.0" />
+    <dimensions x="1.5*m" y="1.0*m" z="1.0*m" />
+    <apperture x="0.5*m" y="0.23*m" />
+    <coil dx="0.1*m" dy="0.03*m" />
+  </detector>
+</detectors>
+
+