Create `version()` function

[peterdesmet]

See #224, cf. https://inbo.github.io/camtrapdp/reference/version.html

• Is public
• Works on:
  • Package (list)
  • Resource (list)
  • Dialect (list)
  • Resource (list)
• Returns version number from $schema.
• If provided $schema is not datapackage.org (e.g. will be the case for Camtrap DP), then dive deeper to find the datapackage.org schema.