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

@@ -17,7 +17,6 @@ import tempfile
 from GRID_LRT.auth.get_picas_credentials import picas_cred as pc
 import GRID_LRT
 from GRID_LRT.auth import grid_credentials
-from subprocess import Popen
 
 from GRID_LRT.safepopen import SafePopen