[FEM] add missing eigenfrequcy calculation for Elmer

- while for CCX we output the eigenfrequency, for Elmer the user had to perform the calculation of a sqrt of the complex result.
This is inconvenient and error-prone and also requires the knowledge where the result is output by Elmer and in what format. (cast me more than an hour to find this out)
Therefore perform the calculation for the user and output the result.

src/Mod/Fem/femsolver/elmer/tasks.py

@@ -28,10 +28,10 @@ __url__ = "https://www.freecadweb.org"
 ## \addtogroup FEM
 #  @{
 
+import cmath
 import os
 import os.path
 import subprocess
-import sys
 from platform import system
 
 import FreeCAD
@@ -166,10 +166,9 @@ class Solve(run.Solve):
     def _updateOutput(self, output):
         if self.solver.ElmerOutput is None:
             self._createOutput()
-        if sys.version_info.major >= 3:
-            self.solver.ElmerOutput.Text = output
-        else:
-            self.solver.ElmerOutput.Text = output.decode("utf-8")
+        # check if eigenmodes were calculated and if so append them to output
+        output = self._calculateEigenfrequencies(output)
+        self.solver.ElmerOutput.Text = output
 
     def _createOutput(self):
         self.solver.ElmerOutput = self.analysis.Document.addObject(
@@ -181,6 +180,62 @@ class Solve(run.Solve):
         self.analysis.addObject(self.solver.ElmerOutput)
         self.solver.Document.recompute()
 
+    def _calculateEigenfrequencies(self, output):
+        # takes the EigenSolve results and performs the calculation
+        # sqrt(aResult) / 2*PI but with aResult as complex number
+
+        # first search the output file for the results
+        OutputList = output.split("\n")
+        modeNumber = 0
+        modeCount = 0
+        real = 0
+        imaginary = 0
+        haveImaginary = False
+        FrequencyList = []
+        for line in OutputList:
+            LineList = line.split(" ")
+            if (len(LineList) > 1) \
+             and (LineList[0] == "EigenSolve:") \
+             and (LineList[1] == "Computed"):
+                # we found a result and take now the next LineList[2] lines
+                modeCount = int(LineList[2])
+                modeNumber = modeCount
+                continue
+            if modeCount > 0:
+                for LineString in reversed(LineList):
+                    # the output of Elmer may vary, we only know the last float
+                    # is the imaginary and second to last float the real part
+                    if self._isNumber(LineString):
+                        if not haveImaginary:
+                            imaginary = float(LineString)
+                            haveImaginary = True
+                        else:
+                            real = float(LineString)
+                            break
+                eigenFreq = complex(real, imaginary)
+                haveImaginary = False
+                # now we can perform the calculation
+                eigenFreq = cmath.sqrt(eigenFreq) / (2 * cmath.pi)
+                # create an output line
+                FrequencyList.append("Mode " + str(modeNumber - modeCount + 1) \
+                    + ": " + str(eigenFreq.real) + " Hz")
+                modeCount = modeCount - 1
+        if modeNumber > 0:
+            # push the results and append to output
+            self.pushStatus("\n\nEigenfrequency results:")
+            output = output + "\n\nEigenfrequency results:"
+            for i in range(0, modeNumber):
+                output = output + "\n" + FrequencyList[i]
+                self.pushStatus("\n" + FrequencyList[i])
+            self.pushStatus("\n")
+        return output
+
+    def _isNumber(self, string):
+        try:
+            float(string)
+            return True
+        except ValueError:
+            return False
 
 class Results(run.Results):