fix test_prevent_dtype_int

[veenstrajelmer]

FAILED tests/test_xarray_helpers.py::test_prevent_dtype_int because the merged file sizes of files retrieved with the new CDS-beta (#739) are different. This happens because ERA5 data is now zipped float instead of scaled int. So in fact, the prevent_dtype_int is not needed anymore for this new data.

However, for compatibility with old (already downloaded) ERA5 netcdf files, do check whether there are no scale_factor (etc) attrs present upon merging, otherwise we get corrupt files like in #239. If these attrs are present, drop them and convert dtype to float