Exclude Glia/Unimportant from :Neuron set

[stuarteberg]

In the config, the neuron-label-criteria setting specifies when to promote a :Segment to :Neuron according to synapse count, status, or the presence of special properties (e.g. somaLocation, type, etc.).

But we should also EXCLUDE some segments based on bad statuses (Unimportant, Glia), even if they would otherwise have qualified as :Neuron.