add method to apply a mask

[esheldon]

We may want to "zero out" all values in a map of a scalar field based on a mask.

The example that came up was when we have a set of maps but we want to restrict them to a boundary region, without modifying the values in the map. Because we don't want to modify the values it isn't just an intersection

[esheldon]

maybe what we want is a method to a map to just intersect but not or intersect

smap = HealSparsMap(...)
bounds = HealSparseMap(...)
smap.intersect(bounds)  # intersect but don't modify the values