Append to zarr redundant with mode='a'

[davidbrochart]

When appending to a zarr store, we need to set append_dim=dim_name but also mode='a'. Any reason to also specify the writing mode? I think it should automatically be set to 'a' if append_dim is not None.

[shoyer]

This sounds reasonable to me.

[davidbrochart]

@shikharsg, @jendrikjoe and @rabernat might be interested too.

[shikharsg]

Looks good to me