FEM: examples and unit tests calculix solver, add ccx cantilever with quad8 elements

src/Mod/Fem/femtest/data/calculix/ccx_cantilever_ele_quad8.inp

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+** written by FreeCAD inp file writer for CalculiX,Abaqus meshes
+** highest dimension mesh elements only.
+
+** Nodes
+*Node, NSET=Nall
+1, 0, 500, 0
+2, 0, 500, 1000
+3, 8000, 500, 0
+4, 8000, 500, 1000
+5, 0, 500, 500
+6, 0, 500, 250
+7, 0, 500, 750
+8, 4000, 500, 0
+9, 2000, 500, 0
+10, 6000, 500, 0
+11, 8000, 500, 500
+12, 8000, 500, 250
+13, 8000, 500, 750
+14, 4000, 500, 1000
+15, 2000, 500, 1000
+16, 6000, 500, 1000
+17, 4000, 500, 500
+18, 2000, 500, 500
+19, 4000, 500, 750
+20, 2000, 500, 750
+21, 6000, 500, 500
+22, 6000, 500, 750
+23, 4000, 500, 250
+24, 6000, 500, 250
+25, 2000, 500, 250
+
+
+** Face elements
+*Element, TYPE=S8, ELSET=Efaces
+1, 1, 5, 17, 8, 6, 18, 23, 9
+2, 5, 2, 14, 17, 7, 15, 19, 18
+3, 17, 14, 4, 11, 19, 16, 13, 21
+4, 8, 17, 11, 3, 23, 21, 12, 10
+
+** Define element set Eall
+*ELSET, ELSET=Eall
+Efaces
+
+
+
+***********************************************************
+** Element sets for materials and FEM element type (solid, shell, beam, fluid)
+*ELSET,ELSET=MechanicalMaterialThickness
+Efaces
+
+***********************************************************
+** constraints fixed node sets
+** ConstraintFixed
+*NSET,NSET=ConstraintFixed
+1,
+2,
+5,
+6,
+7,
+
+***********************************************************
+** Materials
+** see information about units at file end
+** FreeCAD material name: Calculix-Steel
+** MechanicalMaterial
+*MATERIAL, NAME=MechanicalMaterial
+*ELASTIC
+210000,0.3
+
+***********************************************************
+** Sections
+*SHELL SECTION, ELSET=MechanicalMaterialThickness, MATERIAL=MechanicalMaterial
+1000
+
+***********************************************************
+** At least one step is needed to run an CalculiX analysis of FreeCAD
+*STEP
+*STATIC
+
+
+***********************************************************
+** Fixed Constraints
+** ConstraintFixed
+*BOUNDARY
+ConstraintFixed,1
+ConstraintFixed,2
+ConstraintFixed,3
+ConstraintFixed,4
+ConstraintFixed,5
+ConstraintFixed,6
+
+
+***********************************************************
+** constraints force node loads
+*CLOAD
+** ConstraintForce
+** node loads on shape: CanileverPlate:Edge3
+3,2,-1.6653345369377E-10
+3,3,-7.5000000000000E+05
+4,2,-1.6653345369377E-10
+4,3,-7.5000000000000E+05
+11,2,-3.3306690738755E-10
+11,3,-1.5000000000000E+06
+12,2,-6.6613381477509E-10
+12,3,-3.0000000000000E+06
+13,2,-6.6613381477509E-10
+13,3,-3.0000000000000E+06
+
+
+
+***********************************************************
+** Outputs --> frd file
+*NODE FILE, OUTPUT=2d
+U
+*EL FILE
+S, E
+** outputs --> dat file
+** reaction forces for Constraint fixed
+*NODE PRINT, NSET=ConstraintFixed, TOTALS=ONLY
+RF
+
+
+***********************************************************
+*END STEP 
+
+***********************************************************
+** CalculiX Input file
+**   written by    --> FreeCAD 0.20.25343 (Git)
+**   written on    --> Thu Jul 29 13:06:02 2021
+**   file name     --> ccx_cantilever_ele_quad8.FCStd
+**   analysis name --> Analysis
+**
+**
+***********************************************************
+**  About units:
+**  See ccx manual, ccx does not know about any unit.
+**  Golden rule: The user must make sure that the numbers he provides have consistent units.
+**  The user is the FreeCAD calculix writer module ;-)
+**
+**  The unit system which is used at Guido Dhondt's company: mm, N, s, K
+**  Since Length and Mass are connected by Force, if Length is mm the Mass is in t to get N
+**  The following units are used to write to inp file:
+**
+**  Length: mm (this includes the mesh geometry)
+**  Mass: t
+**  TimeSpan: s
+**  Temperature: K
+**
+**  This leads to:
+**  Force: N
+**  Pressure: N/mm^2 == MPa (Young's Modulus has unit Pressure)
+**  Density: t/mm^3
+**  Gravity: mm/s^2
+**  Thermal conductivity: t*mm/K/s^3 == as W/m/K == kW/mm/K
+**  Specific Heat: mm^2/s^2/K = J/kg/K == kJ/t/K
+**