Move SafePopen to its own submodule

Having SafePopen in GRID_LRT.application.submit apparently gives some problems if other (sub)modules that use it are imported before the class is defined in submit.py. This moves it to its own submodule GRID_LRT.safepopen to avoid that.

GRID_LRT/application/submit.py

@@ -300,7 +300,7 @@ class LouiLauncher(JdlLauncher):
 
     def __exit__(self, exc_type, exc_value, traceback):
         return None
-
+        
 
 class SpiderLauncher(JdlLauncher):
     """ Grants users the power of a SPIDER. The SpiderLauncher creates a launch file to be passed to Slurm. It is made for the SPIDER platform offered by Surf and has some (possibly) specific options for that.