3D viewer: add basic SVG catalog export

There are currently no options for the user to change. A two-column layout is
created with a margin of 10 and a padding per image of also 10.
3D viewer: add options dialog for SVG catalog export
Neuron name service: allow creation without default fallback list
Neuron name service: allow null as name if no fallback matches

This allows for external teting if there was a match in the fallback list.
Neuron name service: don't try to fetch data without skeletons
Neuron name service: allow creation of new instances parallel to singleton

This is needed to re-use the neuron name service's functionality in the 3D
viewer's catalog export.
3D viewer: add sorting by different labeling for SVG catalog export

This allows to select an alternative labeling for sorting the skeletons in the
SVG catalog export. For labeling individual results, the general neuron name
service is still in use. However, for sorting (and therefore determining the
cell in the output table) one can now choose a different labeling scheme.
Another neuron name service instance is used for creating the names and
therefore the options are same. In contrast to the regular name service, no
fallback list is used and if a labeling cannot be applied to a neuron (e.g. to
to the lack of annotations with a chosen meta annotation), an error is raised.
Merge branch 'neuron-catalog-svg'

Closes #830