All filters outputting fields should have a default field name

The base class for filters that generate a field, FieldFilter, has as part of its state the name given to the field that is generated. That's all fine, but the default name is a blank string, which is no good. Each filter should pick an intelligent name.

The default name should depend on the filter. For example, the PointElevation filter should name its field something like Elevation. The CellAverage filter should name the output field the same as the input.