Allow specifying the "root" name

[mgeplf]

For the cell-densities cell-density and cell-densities inhibitory_and_excitatory_neuron_densities commands. It defaults to "root", like for the AIBS hierarchy, if not specified.

[codecov-commenter]

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[mgeplf]

I reran cell-density, glia-cell-density and inhibitory-neuron-densities using the pre-commit code, and this code, with the stock AIBS hierarchy. The densities matched for:

    'cell_density.nrrd',

    'astrocyte_density.nrrd',
    'glia_density.nrrd',
    'microglia_density.nrrd',
    'neuron_density.nrrd',
    'oligodendrocyte_density.nrrd',

    'gad67+_density.nrrd',
    'pv+_density.nrrd',
    'sst+_density.nrrd',
    'vip+_density.nrrd',

I then changed the hierarchy root name to "asdf" (and the first_estimates.csv), and reran it; and the above outputted densities were the same.