small changes to NXbeam and NXmonochromator

# Conflicts:
#	base_classes/NXbeam.nxdl.xml
#	base_classes/nyaml/NXbeam.yaml
#	base_classes/nyaml/NXmonochromator.yaml

base_classes/NXmonochromator.nxdl.xml

@@ -75,6 +75,16 @@
              Wavelength dispersion at the exit slit.
         </doc>
     </field>
+    <group name="entrance_slit" type="NXaperture">
+        <doc>
+             Size, position and shape of the entrance slit of the monochromator.
+        </doc>
+    </group>
+    <group name="exit_slit" type="NXaperture">
+        <doc>
+             Size, position and shape of the exit slit of the monochromator.
+        </doc>
+    </group>
     <group type="NXdata" name="distribution"/>
     <group type="NXgeometry" name="geometry" deprecated="Use the field `depends_on` and :ref:`NXtransformations` to position the monochromator and NXoff_geometry to describe its shape instead"/>
     <group type="NXoff_geometry" minOccurs="0">