[FEM] fix solver console bugs

- for Elmer and Z88 on Windows several windows pop up (console windows). This is maybe annoying and the user is wondering what is going on, but the main problem is that when you close them, you break the solving process.
Therefore, on Windows only, hide the empty popup windows.

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

@@ -121,10 +121,18 @@ class Solve(run.Solve):
                 if os.path.isdir(solvpath):
                     os.environ["ELMER_HOME"] = solvpath
                     os.environ["LD_LIBRARY_PATH"] = "$LD_LIBRARY_PATH:{}/modules".format(solvpath)
-            self._process = subprocess.Popen(
+            # hide the popups on Windows
+            if system() == "Windows":
+                self._process = subprocess.Popen(
                 [binary], cwd=self.directory,
                 stdout=subprocess.PIPE,
-                stderr=subprocess.PIPE)
+                stderr=subprocess.PIPE,
+                startupinfo=femutils.startProgramInfo("hide"))
+            else:
+                self._process = subprocess.Popen(
+                    [binary], cwd=self.directory,
+                    stdout=subprocess.PIPE,
+                    stderr=subprocess.PIPE)
             self.signalAbort.add(self._process.terminate)
             output = self._observeSolver(self._process)
             self._process.communicate()

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

@@ -33,6 +33,7 @@ import os
 import os.path
 import subprocess
 import tempfile
+from platform import system
 
 from FreeCAD import Console
 from FreeCAD import Units
@@ -223,7 +224,12 @@ class Writer(object):
                     unvPath,
                     "-scale", "0.001", "0.001", "0.001",
                     "-out", self.directory]
-            subprocess.call(args, stdout=subprocess.DEVNULL)
+            # hide the popups on Windows
+            if system() == "Windows":
+                subprocess.call(args, stdout=subprocess.DEVNULL,
+                    startupinfo=femutils.startProgramInfo("hide"))
+            else:
+                subprocess.call(args, stdout=subprocess.DEVNULL)
 
     def _writeStartinfo(self):
         path = os.path.join(self.directory, _STARTINFO_NAME)