Enable using url source as a stream

[sandorkertesz]

This PR enables using GRIB data from a URL or multiple URLs as a stream.

import earthkit.data

ds = earthkit.data.from_source("url",
                       ["https://get.ecmwf.int/repository/test-data/earthkit-data/examples/test.grib",
                        "https://get.ecmwf.int/repository/test-data/earthkit-data/examples/test4.grib"], 
                        stream=True, batch_size=3)

for f in ds:
    # f is a fieldlist
    print(type(f))
    print(f"len={len(f)}")
    for g in f:
        print(f" {g}")

<class 'earthkit.data.readers.grib.memory.GribFieldListInMemory'>
len=3
 GribField(2t,None,20200513,1200,0,0)
 GribField(msl,None,20200513,1200,0,0)
 GribField(t,500,20070101,1200,0,0)
<class 'earthkit.data.readers.grib.memory.GribFieldListInMemory'>
len=3
 GribField(z,500,20070101,1200,0,0)
 GribField(t,850,20070101,1200,0,0)
 GribField(z,850,20070101,1200,0,0)

Example: https://earthkit-data.readthedocs.io/en/develop/examples/grib_url_stream.html