We *really* need an entity_export function

[noblem]

There is no matching export function for entity_import ... among it's most obvious utility as a way of extracting data, it would also give us another way of counting sample set sizes. But that should also be doable as a low-level API and high-level CLI command in FISS ... as requested in #81

[noblem]

Done in v0.16.18: not as a general entity_export, though, but rather as

• pset_export
• sset_export
• ptset_export

which are slightly easier to use (and follows legacy FISS usage)