Add a doc for EMCalorimeterSoLID

docs/EMCalorimeterSoLID.rst

+SoLID ElectroMagnetic Calorimeter
+=========
+
+A electromagnetic calorimeter for the SoLID detector. It is a ring-shape detector with hexgonal modules.
+
+Example use:
+
+.. code-block:: XML
+
+    <detector id="3" name="LAECPreShower" type="EMCalorimeterSoLID" readout="LAEC_PrShHits" vis="PurpleVis" >
+      <comment>Large Angle Electromagnetic Calorimeter (LAEC) Preshower for SIDIS</comment>
+      <dimensions rmin="83*cm" rmax="140*cm" z0="-65*cm" rmod="6.25*cm" rtol="1.0*cm" />
+      <layer repeat="1" vis="InvisibleWithDaughters">
+        <slice material = "Lead"  thickness = "1.12*cm" vis="BlueVis"/>
+        <slice material = "EJ204" thickness = "6.25*cm" vis="PurpleVis" sensitive="yes"/>
+      </layer>
+    </detector>
+
+    <detector id="4" name="LAECShower" type="EMCalorimeterSoLID" readout="LAEC_ShHits" vis="PurpleVis" >
+      <comment>Large Angle Electromagnetic Calorimeter (LAEC) Shower for SIDIS</comment>
+      <dimensions rmin="83*cm" rmax="140*cm" z0="-57*cm" rmod="6.25*cm" rtol="1.0*cm" />
+      <layer repeat="194" vis="GreenVis">
+        <slice material = "Lead"  thickness = "0.05*cm"/>
+        <slice material = "EJ204" thickness = "0.15*cm" sensitive="yes" limits="cal_limits"/>
+        <slice material = "Air"   thickness = "0.012*2*cm"/>
+      </layer>
+    </detector>
+
+Detector description variables
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+dimensions
+    Dimensions of the sensitive detector
+
+    rmin
+        Inner radius of the detector
+    rmax
+        Outer raidus of the detector
+    z0
+        Z position (along the beam line) of the front surface of the detector
+    rmod
+        Outer radius (side length) of the hexgonal module
+    rtol
+        Extrusion Tolerance for putting modules inside the detector ring
+
+layer
+    Layer definition of the hexgonal modules
+
+    repeat
+        Number of the same slices to stack along z
+    slice
+        Slice definition in the layer
+
+        material
+            slice material
+        thickness
+            slice thickness
+