Add another "single state" variable to ND-tree node

[HactarCE]

Currently there is a field, single_state: Option<u8> that holds None if the node contains more than one cell state value or Some(state) if all cells in the node have state state.

I'd like to retype this field to the enum:

enum CellsSummary {
    /// All cells have the same state.
    SingleState(u8),
    /// There is a mix of zero and nonzero cells; all nonzero cells have the same state.
    SingleNonzeroState(u8),
    /// There are multiple nonzero cell states.
    Mixed,
}

This will enable rendering while zoomed to can include colors even in sparsely populated regions.