refactor summary statistics from struct to function

[musoke]

Currently, summary statistics are extracted by passing a list of structs that have constructors with the appropriate signature. Headers for the resulting CSV are constructed separately and archanely.
Attempting to describe this succinctly in the docs has made it clear that it is not great.

Simplify this by accepting a list of functions instead, each of which returns both data and a header.
The return type could be

• a tuple (::Vector{String}, ::Vector{Float64}).
  This would be simple, but possibly too ad hoc.

• a Dataframe.
  This would be a new, possibly undesirable dependency.

[musoke]

Depends on #107