Event Validation - Position

[upsonp]

When validating an event at sea the validation will expect an event to have at least one action and that action must have at least a date and a sounding, but Dart doesn't validate that the event has a position.

The easiest place to fix a missing position is going to be at sea when data is being loaded, it'll be much harder to fix once users get to the BioChem loading process #147

In the core.validation.validate_event() function when checking an action exists and that it has a date and depth also check for a position.