Feature/#4 adding a cron job to refresh vocabularies

[jarno-knaw]

Closes #4.

Add a cron container which loads vocabularies based on the yml files in the data directory every hour.

Also added a configuration option for using this feature for refreshing existing vocabularies:

config:
  refresh: Yes
  refreshInterval: 24 # Time in hours since last update
  type: fetch
  location: https://wherever-your-data-is/file.config

Omitting the refreshInterval but setting refresh to True results in refreshing every hour or when the container is restarted, as the interval is set to 0 hours.

[menzowindhouwer]

SPARQLWrapper looks nice :-)

[menzowindhouwer]

some other cron approaches, e.g. in TB2, left me with the impression that cron in docker was complicated, nice to see that it doesn't have to be ...

[menzowindhouwer]

Why do we need a sleep of 10? can we somehow detect if the event happened that we're waiting on?