Add default font to documentation
Add default font to documentation

On a headless Debian installation, this font is missing, which results in matplotlib images without any text on them.
juliaeis/oggm,bearecinos/oggm,OGGM/oggm,anoukvlug/oggm,bearecinos/oggm,TimoRoth/oggm,TimoRoth/oggm,anoukvlug/oggm,OGGM/oggm,juliaeis/oggm