Adding `\articleclass` to differentiate between preprint, letter and final article template

New argument makes the main file easier to use in order to differentiate between preprint, final version and letter (data management letter).

This MR is a relatively big change.

final.def

@@ -5,10 +5,10 @@
 		\begin{flushleft}%
 			\normalsize\sffamily%
 			{\normalsize\MakeUppercase{\@articletype}\par}
-			\ifletter
+			\ifx\@articleclass\letter
 			\marginnote{\includegraphics[width=\linewidth]{./logos/inggrid.pdf}}%[.5\baselineskip]
 			\else
-			\iffinal
+			\ifx\@articleclass\final
 			\marginnote{\includegraphics[width=\linewidth]{./logos/inggrid.pdf}}%[.5\baselineskip]
 			\else
 			\marginnote{\includegraphics[width=\linewidth]{./logos/inggrid-preprints.pdf}}%[.5\baselineskip]
@@ -30,13 +30,13 @@
 		\vskip 2ex
 	}
     \sideinfofield{Date Submitted}{datesubmitted}
-	\ifletter
+	\ifx\@articleclass\letter
 	\sideinfofield{Date Published}{datepublished}
 	\sideinfofield{DOI}{doi}
 	\sideinfofield{License}{licenses}
     \sideinfofield{Keywords}{keywords}
 	\else
-	\iffinal
+	\ifx\@articleclass\final
 	\sideinfofield{Date Received}{datereceived}
 	\sideinfofield{Date Accepted}{dateaccepted}
 	\sideinfofield{Date Published}{datepublished}