Add documentation for CalculateMonteCarloAbsorption

Refs #20787

docs/source/algorithms/CalculateMonteCarloAbsorption.rst

+.. algorithm::
+
+.. summary::
+
+.. alias::
+
+.. properties::
+
+
+Description
+-----------
+
+This algorithm calculates the absorption factors, required for the Paalman Pings method of absorption corrections, using a Monte Carlo procedure. Currently only the acc and ass factors are calculated.
+
+CalculateMonteCarloAbsorption subsequently calls the :ref:`algm-SimpleShapeMonteCarloAbsorption` algorithm for the calculation of each absorption factor.
+
+There are three existing Shape Options: *Flat Plate*, *Annulus* and *Cylinder*. Each shape is defined by a different set of geometric parameters.
+
+Flat Plate parameters: *SampleThickness* and *SampleWidth* for the Sample, *ContainerFrontThickness* and *ContainerBackThickness* for the container.
+Annulus parameters: *SampleInnerRadius* and *SampleOuterRadius* for the Sample, *ContainerInnerRadius* and *ContainerOuterRadius* for the container.
+Cylinder parameters: *SampleRadius* for the sample, *ContainerInnerRadius* and *ContainerOuterRadius* for the container.
+
+The location and orientation of the sample can be defined with *SampleCenter* and *SampleAngle*.
+
+
+Workflow
+--------
+
+
+Usage
+-----
+
+**Example - CalculateMonteCarloAbsorption**
+
+.. testcode:: QENSCalculateShapeMonteCarloAbsorption
+
+    sample_ws = CreateSampleWorkspace(Function="Quasielastic",
+                                      XUnit="Wavelength",
+                                      XMin=-0.5,
+                                      XMax=0.5,
+                                      BinWidth=0.01)
+
+    container_ws = CreateSampleWorkspace(Function="Quasielastic",
+                                         XUnit="Wavelength",
+                                         XMin=-0.5,
+                                         XMax=0.5,
+                                         BinWidth=0.01)
+
+    corrections = CalculateMonteCarloAbsorption(SampleWorkspace = sample_ws,
+                                                SampleChemicalFormula = 'H2-O',
+                                                SampleDensityType = 'Mass Density',
+                                                SampleDensity = 1.0,
+                                                ContainerWorkspace = container_ws,
+                                                ContainerChemicalFormula = 'Al',
+                                                ContainerDensityType = 'Mass Density',
+                                                ContainerDensity = 1.0,
+                                                EventsPerPoint = 200,
+                                                BeamHeight = 3.5,
+                                                BeamWidth = 4.0,
+                                                Height = 2.0,
+                                                Shape = 'FlatPlate',
+                                                SampleWidth = 1.4,
+                                                SampleThickness = 2.1,
+                                                ContainerFrontThickness = 1.2,
+                                                ContainerBackThickness = 1.1)
+
+    ass_ws = corrections[0]
+    acc_ws = corrections[1]
+
+    print("Workspaces: " + str(ass_ws.getName()) + ", " + str(acc_ws.getName()))
+    print("Y-Unit Label of " + str(ass_ws.getName()) + ": " + str(ass_ws.YUnitLabel()))
+    print("Y-Unit Label of " + str(acc_ws.getName()) + ": " + str(acc_ws.YUnitLabel()))
+
+.. testcleanup:: QENSCalculateShapeMonteCarloAbsorption
+
+    DeleteWorkspace(sample_ws)
+    DeleteWorkspace(container_ws)
+    DeleteWorkspace(corrections)
+
+**Output:**
+
+.. testoutput:: QENSCalculateShapeMonteCarloAbsorption
+
+    Workspaces: corrections_ass, corrections_acc
+    Y-Unit Label of corrections_ass: Attenuation factor
+    Y-Unit Label of corrections_acc: Attenuation factor
+
+.. categories::
+
+.. sourcelink::
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