Add options to allow (easier) use for posters that aren't for thesis or Clinic

Right now, we define a bunch of things to create the header and footer that expect the poster to be either a thesis poster (in which case we put the class name and number at the head of the header, and maybe put an advisor or reader in the footer) or a Clinic poster (where we format the header and footer otherwise).

To make the poster class more generic (i.e., more suitable for summer research posters and the like), we need to add an additional option or have some fallback formatting so that the headers and footers work better.

For example, with a summer poster using the thesis DCO, we can adjust things as follows to make the header smaller as well as to not include the class line at all:

\geometry{%
headheight=4.5in,               % Make height of header shorter.
headsep=1in,                    % Increase space between bottom of
                                % header and rest of page.
footskip=1in,
}

%%% Redefine the header to not include code to do anything with
%%% \@class (thus saving a line's worth of vertical space).
\makeatletter
\fancyhead[C]{%
  \begin{center}
    % {\color{black}\Large\textsf{\@class}\\}%
    {\color{hmcorange}\fontsize{100pt}{120pt}\selectfont\textsf{\textbf{\@title}}\\[0.25em]}
    {\color{black}\Large\textsf{\@author}\\[-0.5em]}
  \end{center}
}
\makeatother

We also needed to comment out the \advisor, \reader, and \class commands to avoid defining them (and thus causing extra text to appear in the footer).

Probably we want to do some or all of the following:

Define separate default footers and headers when clinic or thesis aren't provided as DCOs
Warn if \class, \advisor, \reader, \sponsor, \sponsorlogo, or \sponsorlogowidth (and maybe \posteryear) are defined and direct the user to include the appropriate DCO

harveymuddcollege / hmcposter-class

Add options to allow (easier) use for posters that aren't for thesis or Clinic #3