Rename and reorganise files

[nikosbosse]

• convencience-functions.R should be renamed to transform_forecasts.R
• set_forecast_unit() should be moved to a file get_forecast_unit.R (maybe forecast_unit.R?) together with get_forecast_unit().
• correlations.R should be get_correlations.R
• different get_-functions should be moved out of get_-functions.R
• metrics-range.R --> metrics-interval_range.R
• customise_metric() should maybe get its own file. Alternatively, the corresponding tests should be moved.
• the current function validate_scores should really be assert_scores()

[seabbs]

Is this all done now?

[nikosbosse]

We did another set of renaming/restructuring files previously, this one is still open.

[sbfnk]

get_-functions.R also contains some assert_... and test_... functions which I think should be moved to one of the check-....R fiiles.

[jamesmbaazam]

@nikosbosse You could perhaps do this as a final step before release since it's not a functional change?