use geometry package

Many of the templates, e.g., [titlepage.tex](https://github.com/acl-org/ACLPUB/blob/master/templates/titlepage.tex) have the following in their preamble:
```
\setlength\topmargin{0.2cm} \setlength\oddsidemargin{-0cm}
\setlength\textheight{24.7cm} \setlength\textwidth{16cm}
\setlength\columnsep{0.6cm} \newlength\titlebox \setlength\titlebox{2.00in}
\setlength\headheight{5pt} \setlength\headsep{0pt}
\setlength\footskip{1.0cm}
\setlength\leftmargin{0.0in}
```
Could we replace this with the geometry package? I think it would be simpler to understand that way:
```
\usepackage[a4paper,margin=2.5cm,columnsep=0.6cm,headheight=5pt,headsep=0pt,footskip=1cm]{geometry}
\newlength\titlebox \setlength\titlebox{5cm}
```