Automatically detect different legacy file formats

[TomNicholas]

We should design the backends so that the user doesn't have to specifically state which type of file they are trying to open. I believe pangeo-forge has some logic for detecting the file format automatically.

Original kerchunk gripe here: fsspec/kerchunk#376

[TomNicholas]

The closest thing pangeo-forge has to this is here. That code abstracts over the different kerchunk openers, but I don't think it automatically detects the correct one to use.

[TomNicholas]

This was partially closed by #43, which can auto-detect the difference between netCDF3 and netCDF4 at least.

[TomNicholas]

Copying Ryan's suggestion here so we don't forget to try it:

You can do this without opening the file with netCDF at all. Both file types have a "magic" at the beginning of the file.

def guess_file_type(fp) -> FileType:
    magic = fp.read(4)
    fp.seek(0)
    if magic[:3] == b"CDF":
        return FileType.netcdf3
    elif magic == b"\x89HDF":
        return FileType.hdf5
    else:
        raise ValueError(f"Unknown file type - magic {magic}")

[TomNicholas]

I think this was closed by #143