Add commentstring to vim editor

Add a commentstring to the vim syntax, such that block comments are easily made using gcc

Author: Lomholy

support/common/editors/mccode.vim

@@ -12,7 +12,7 @@
 
 " McStas/McXtrace instruments are mostly c syntax so start with that
 :runtime! syntax/c.vim
-
+:setlocal commentstring=//\ %s
 "keywords for mcstas statements
 :syntax keyword instrStatement DECLARE DEPENDENCY DEFINE END FINALLY INITIALIZE MCDISPLAY SAVE SHARE USERVARS
 :syntax keyword instrStatement TRACE DEFINITION PARAMETERS POLARISATION SETTING STATE