put SolutionFile class in an own file

LDDsimulation/solutionFile.py

+import dolfin as df
+# https://github.com/geo-fluid-dynamics/phaseflow-fenics
+# adapted from Lucas Ostrowski
+class SolutionFile(df.XDMFFile):
+    """
+    This class extends `df.XDMFFile` with some minor changes for convenience.
+    """
+    def __init__(self, mpicomm, filepath):
+        df.XDMFFile.__init__(self, mpicomm, filepath)
+
+        self.parameters["functions_share_mesh"] = True
+        self.parameters["flush_output"] = False
+        self.path = filepath # Mimic the file path attribute from a `file` returned by `open`