Docs for Roman pot + fixes to pip requirements

It's not fully finished, but you should merge so people can compile docs locally.

docs/RomanPot.rst

+Roman Pot
+=========
+
+A pair of Roman Pot-style detectors for detection of particles scattered by very small angles.
+
+Example use:
+
+.. code-block:: XML
+
+   <detector id = "1" name = "MyRomanPot" type = "RomanPot" readout =
+              "ForwardRomanPotHits" vis = "RedVis">
+      <dimensions x = "3.0*cm" y = "3.0*cm" delta = "0.005*cm" />
+      <frame x = "10.0*cm" y = "5.0*cm" z = "2*cm" />
+      <position z_offset = "0.0*m" rotation = "false" vmax = "10*cm" v = "2.0*cm" />
+      <layer repeat = "5">
+        <slice material = "Carbon"  thickness = "0.5*mm"  vis = "BlueVis" />
+        <slice material = "Silicon" thickness = "0.03*cm" vis = "GreenVis" sensitive = "true"  />
+        <slice material = "Carbon"  thickness = "0.5*mm"  vis = "BlueVis" />
+        <slice material = "Vacuum"  thickness = "1.0*mm"  vis = "InvisibleWithDaughters" />
+      </layer>
+    </detector>
+
+Detector description variables
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+dimensions
+    Dimensions of the sensitive detector.
+
+    x
+        Width of the detector.
+    y
+        Height of the detector.
+    delta
+        Separation gap - the distance between the inner edges of the active area substrate.
+
+frame
+    Dimensions of the frame that houses the sensitive detector.
+
+    x
+        Width of the frame.
+    y
+        Height of the frame.
+    z
+        thickness of the frame.
+
+position
+Positioning of the whole detector pair relative to the beamline.
+
+z_offset
+    Distance along the beamline.
+
+    rotation
+        Boolean which determines whether the detector pair is horizontal (false) or vertical (true).
+    vmax
+        Furthest distance of the detector from the beam (used to create the full unit unclosure).
+    v
+        Current insertion depth of the detectors (distance of the inner edge relative to the beam axis).
+
+layer
+    Material stack used for the sensitive detector.
+
+    repeat
+        Number of layers in a single detectors.