rgho 3.0.2

This new version of rgho now uses the most recent API of the WHO,
which simplifies considerably the package, and relies on the package
ODataQuery.

Breaking changes

• Objects created with get_gho_* are now of class "gho" and "data.frame"
• Former lower-level access to the API do not work anymore
• If a connection error occurs, the function returns an empty list with a
  message as argument
• The proxy settings and the number of retries are now managed by ODataQuery
  package.
• There is no memoisation anymore
• get_gho_codes() is superseded by get_gho_values() to better reflect the
  new API terminology