investigate possibility to tweak the reader for WV to let user have access to one or all imagette within a SAFE

[agrouaze]

idea is to align the behavior with ENVISAT ASAR WVI reader.
see https://github.com/umr-lops/xarray-n1-asar/blob/418dd9b99d61b1b96abeab6f8aea847bd6ef62aa/n1_asar/asa_wvi_1p_reader.py#L23