Add support for VPL_PRECIOUS_FILES

As hard-coded PRECIOUS_FILES, VPL_PRECIOUS_FILES can be used to keep files for a longer time during a run.
VPL_PRECIOUS_FILES can be set in vpl_common_script.sh for example

Basic_System/ef/vpl_execution.sh

@@ -6,6 +6,8 @@ fi
 
 # Files to keep until the very last moment
 PRECIOUS_FILES="vpl_custom_setup.sh vpl_custom_run.sh"
+# VPL_PRECIOUS_FILES can be set in vpl_common_script.sh
+# if some files need to be kept until vpl_custom_setup.sh
 
 error_message() {
     message=$1
@@ -71,7 +73,7 @@ if [ "$MODE" != evaluate ]
 then
     for file in *
     do
-        if ! search $file $VPL_SUBFILES $PRECIOUS_FILES
+        if ! search $file $VPL_SUBFILES $PRECIOUS_FILES $VPL_PRECIOUS_FILES
         then
             rm -fr $file
         fi