Provide helper functions for poster header composition

[odeleongt]

The poster header area is probably the most variable in terms of customization requirements. Several important pieces of information can be included there.

Helper functions should be provided to easily customize the most common elements, including poster title, author lists and affiliations, contact information for main author and logos.

It should be straightforward to define the arrangement of these elements, ideally with a sensible default and options to modify values in the params tag of the YAML header.