Improve printing for multiclass neuronlist

[jefferis]

#' "1702336197", "1793744512", "1976565858", "2007578510", "2101339904",
#' "5813003258", "2069647778", "1947192569", "1883788812", "1916485259",
#' "1887177026", "2101348562", "2132375072", "2256863785", "5813002313",
#' "5813054716", "5813018847", "5813055448", "1763037543", "2101391269",
#' "1794037618", "5813018729", "2013333009")
#'
#' # Get neurons
#' neurons = neuprintr::neuprint_read_neurons(al.local.neurons)

> neurons.split
'neuronlist' containing 28 'neuronneuprintneuron' objects of classes
neuprintneuron         neuron 
             3             25 
and 'data.frame' with 14 vars [30.3 MB]