FEM: Make sure .inp doesn't contain force values that crash ccx

ccx crashes when a force has too many digits like this:
FemConstraintForce,2,1.5966711853290134e-18
but it's OK with that format:
FemConstraintForce,2,1.5966711853e-18

An example of ccx error:
*ERROR reading *CLOAD. Card image:
FEMCONSTRAINTFORCE,3,1.5966711853290134E-18

This commits adds formatting to make sure the numbers are within ccx
limits.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>

src/Mod/Fem/MechanicalAnalysis.py

@@ -543,9 +543,12 @@ def write_calculix_input_file(self):
                 vec = ForceObject['Object'].DirectionVector
                 inpfile.write('*CLOAD\n')
                 inpfile.write('** force: ' + str(ForceObject['NodeLoad']) + ' N,  direction: ' + str(vec) + '\n')
-                inpfile.write(ForceObject['Object'].Name + ',1,' + repr(vec.x * ForceObject['NodeLoad']) + '\n')
-                inpfile.write(ForceObject['Object'].Name + ',2,' + repr(vec.y * ForceObject['NodeLoad']) + '\n')
-                inpfile.write(ForceObject['Object'].Name + ',3,' + repr(vec.z * ForceObject['NodeLoad']) + '\n\n')
+                v1 = "{:.15}".format(repr(vec.x * ForceObject['NodeLoad']))
+                v2 = "{:.15}".format(repr(vec.y * ForceObject['NodeLoad']))
+                v3 = "{:.15}".format(repr(vec.z * ForceObject['NodeLoad']))
+                inpfile.write(ForceObject['Object'].Name + ',1,' + v1 + '\n')
+                inpfile.write(ForceObject['Object'].Name + ',2,' + v2 + '\n')
+                inpfile.write(ForceObject['Object'].Name + ',3,' + v3 + '\n\n')
 
         # write outputs, both are needed by FreeCAD
         inpfile.write('\n** outputs --> frd file\n')