Add check for clearing log only on linux

The log file cannot be cleaned when on Windows, because due
to file locks, cannot open file when it is being used by Mantid
to write to it.

The best solution for now is to clean the file when on Unix system.
On windows the log file will still be produced, but will contain the logs
of all times the routines were run.

src/mvesuvio/main/run_routine.py

@@ -19,6 +19,7 @@
 import sys
 import dill         # To convert constraints to string
 import re
+import os
 
 
 class Runner:
@@ -83,8 +84,10 @@ def run(self):
             return
 
         # Erase previous log
-        with open(self.mantid_log_file, 'w') as file:
-            file.write('')
+        # Not working on Windows due to shared file locks
+        if os.name == 'posix': 
+            with open(self.mantid_log_file, 'w') as file:
+                file.write('')
 
         # If any ws for y fit already loaded
         wsInMtd = [ws in mtd for ws in self.ws_to_fit_y_space]  # Bool list