handle nulls in k_ring_smoothing

[benmcdermott]

If you apply k_ring_smoothing on a dataframe that contains nulls (created by e.g. geo_to_h3_aggregate), then the nulls are mapped to 0. This is probably unwanted behaviour.

Could nulls be handled in this function by doing something like this?

• returning an aggregated value where the k-ring is completely non-null (current method)
• returning an aggregated value where there are a mix of nulls and non-nulls in the k-ring
• returning null where all k-ring vals are null