`reduce_spatial_dims` in some metrics defaults to `['latitude','longitude']`

[aaTman]

All good! I am hesitant to do so mainly because this creates a default for these specific metrics that will be very expensive and produce fundamentally different results.

In other words, the default for these metrics should reduce the spatial dimensions based on some of the answered science questions. Maybe this warrants a slightly different API or pattern? It's not the end of the world if it requires an extra 20 characters for each time these metrics are called though.

Originally posted by @aaTman in #292 (comment)

[linear]

EWB-89