[feat] Forward keyword arguments from `open_dataset` to `MetopDataset`

[lupemba]

Forward arbitrary keyword arguments from open_dataset to MetopDataset

Currently MetopDataset supports the following arguments
MetopDataset(file_path::AbstractString; auto_convert::Bool = true, high_precision::Bool=false, maskingvalue = missing)
but only maskingvalue value is implemented in open_dataset

[lupemba]

closed with #16