Fixes an incompatibility between jfp1.cls and the 'relsize' package.
Justification for the fix:
1. `pdflatex` fails on this document
```
\documentclass{jfp1}
\begin{document}
\maketitle
hello {\Large world}
\end{document}
```
2. `pdflatex` succeeds on this document
```
\documentclass{jfp1}
\renewcommand\Large{\@setfontsize\@xvpt{18}}
\begin{document}
\maketitle
hello {\Large world}
\end{document}
```
I do not know why (1) fails.
The new line in (2) is copied from `jfp1.cls`.
