Rename `available_forecasts()` to `get_forecast_counts()`

[nikosbosse]

Updated description:

• Rename the function available_forecasts() to get_forecast_counts().
• Rename the class that is used for plotting to forecast_counts.

This ensures that the function is named in line with other functions that use the get_ prefix to signal to the user that they provide helpful additional info based on the data.

[nikosbosse]

@sbfnk do you have suggestions for the name? available_forecasts() is the function that tells you how many forecasts you have. We now have quite a few get_ functions that give you some kind of information.

[sbfnk]

I think I'd call it count_forecasts() or perhaps even just count() if example_quantile is of class forecast. Depending on how tidyverse you want to be could even overlaod dplyr::count() (though would involve taking on dplyr as a dependency, similar to the argument around summarise()).

[nikosbosse]

Argument in favour of get_ something would be consistency with other functions that are called get_something() and which get you some information about the data.