Add missing docstrings to opusapi.py

[jocu8995]

docstrings needed for:

get_img_url function
get_metadata function
create_files_request function
create_images_request function
unpack_json_response function
get_radial_res_query function
get_time_query function
get_between_resolutions function
get_dataframe_from_meta_dic function
OPUSImageURL class requires parameter in docstring
OPUS class requires parameter in docstring
init functions