add ForceElement example as integration test

- see #100

tests/examples/force_element/definition.json

+{
+  "executable": [
+    "examples",
+    "ae108-examples-ForceElement"
+  ],
+  "ae108_output": [
+    "force_element.ae108"
+  ],
+  "compare_stdout": "text",
+  "mpi_processes": [
+    1
+  ]
+}
\ No newline at end of file

tests/examples/force_element/references/force_element.vtu

+<?xml version="1.0"?>
+<VTKFile type="UnstructuredGrid" version="0.1" byte_order="LittleEndian" header_type="UInt32" compressor="vtkZLibDataCompressor">
+  <UnstructuredGrid>
+    <Piece NumberOfPoints="4" NumberOfCells="2">
+      <PointData Scalars="result[0]">
+        <DataArray type="Float64" Name="result[0]" format="ascii" RangeMin="0" RangeMax="2.0000000000000004">
+          0 0 2.0000000000000004 0
+        </DataArray>
+        <DataArray type="Float64" Name="result[1]" format="ascii" RangeMin="0" RangeMax="2.0000000000000004">
+          0 0 2.0000000000000004 0
+        </DataArray>
+      </PointData>
+      <CellData>
+      </CellData>
+      <Points>
+        <DataArray type="Float64" Name="Points" NumberOfComponents="3" format="ascii" RangeMin="0" RangeMax="1.4142135623730951">
+          0 0 0 1 0 0
+          1 1 0 0 1 0
+          <InformationKey name="L2_NORM_RANGE" location="vtkDataArray" length="2">
+            <Value index="0">
+              0
+            </Value>
+            <Value index="1">
+              1.4142135624
+            </Value>
+          </InformationKey>
+        </DataArray>
+      </Points>
+      <Cells>
+        <DataArray type="Int64" Name="connectivity" format="ascii" RangeMin="0" RangeMax="3">
+          0 1 2 3 2
+        </DataArray>
+        <DataArray type="Int64" Name="offsets" format="ascii" RangeMin="4" RangeMax="5">
+          4 5
+        </DataArray>
+        <DataArray type="UInt8" Name="types" format="ascii" RangeMin="2" RangeMax="9">
+          9 2
+        </DataArray>
+      </Cells>
+    </Piece>
+  </UnstructuredGrid>
+</VTKFile>