def only_numerics
      numeric_indexes =
        each_with_index
        .select { |v, _i| v.is_a?(Numeric) || v.nil? }
        .map(&:last)

      self[*numeric_indexes]
    end
def only_numerics
      numeric_indexes =
        each_with_index
        .select { |v, _i| v.is_a?(Numeric) || v.nil? }
        .map(&:last)

      self[*numeric_indexes]
    end
Returns a Vector with only numerical data. Missing data is included
 but non-Numeric objects are excluded. Preserves index.