Create abstract forecast class

[nikosbosse]

I wonder if there should be a forecast abstract class, from which forecast_binary, forecast_point, etc. would inherit. In the docs and exported functions (e.g., is_forecast(), validate_forecast()), things appear as if there is such as class, and the actual class forecast_binary, forecast_point, etc. as not so visible.

Originally posted by @Bisaloo in #791 (review)