fix zero buffer for cmems datasets without step attribute

[veenstrajelmer]

from dfm_tools.download import copernicusmarine_get_buffer
dataset_id = 'cmems_obs-oc_glo_bgc-transp_my_l3-multi-4km_P1D'
buffer = copernicusmarine_get_buffer(dataset_id)
print(f"buffer: {buffer}")

Results in a zero buffer since the step attribute is nonexistent in this dataset. Find a more robust way to define the buffer, or make the default 0.5 instead of 0