Adding doco for MaterialRealCMMAux. Closes #376.

doc/content/source/auxkernels/MaterialRealCMMAux.md

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+# MaterialRealCMMAux
+
+!syntax description /AuxKernels/MaterialRealCMMAux
+
+## Description
+
+`MaterialRealCMMAux` can be used to populate an `AuxVariable` with an element of
+a material property that is of type, ColumnMajorMatrix (CMM). In MASTODON, a
+common use case is accessing the deformation or force CMMs from link elements
+when the [`LinearSpring`](source/materials/LinearSpring.md) material or any of
+the seismic isolator materials (e.g., [`ComputeFPIsolatorElasticity`](source/materials/ComputeFPIsolatorElasticity.md)) is used. This `AuxKernel` and the corresponding
+`AuxVariable` should be block-restricted to blocks where the material property
+given by the `property` input parameter below is defined.
+
+For example, the following input file syntax will create the AuxVariable field
+called `global_force_x` with the X direction force in the spring element modeled
+using the LinearSpring material. The AuxKernel is defined as follows:
+
+!listing test/tests/auxkernels/materialrealcmm/spring_static.i block=AuxKernels
+
+In this input, the AuxKernel takes the element (0, 0) from the CMM material
+property named, "ls_global_forces". This material property name refers to the
+global forces in the linear spring material. (See LinearSpring.C for other
+material properties.) In this example, the linear spring material is only defined
+in block '0' and therefore this AuxKernel and AuxVariable are also restricted to
+this block.
+
+!syntax parameters /AuxKernels/MaterialRealCMMAux
+
+!syntax inputs /AuxKernels/MaterialRealCMMAux
+
+!syntax children /AuxKernels/MaterialRealCMMAux