Unify loading of Standard Morphology Files.

[thorstenhater]

All morphology loaders now return a loaded_morphology object comprising

• morphology: the morphology corresponding to the data on disk
• segment_tree: the raw segment tree
• label_dict: all labels defined in the file
  • asc/swc: axon, soma, dend, and apic based on tag 1, 2, 3, 4.
  • nml: the union of the label dicts contained in the metadata, prefixed by grp:, nmd:, and seg:.
• metadata: a set of items given by the fileformat
  • swc: nothing
  • asc: a list of spines and marker sets
  • nml:
    • named_segments a label dict
    • segment_groups a label dict name -> segment ids
    • group_segments a map from segment id to group name(s)
    • segments a label dict, one entry per id
    • cell_id id of the cell defining this morphology, if any
    • id id of the morphology

Note that loading neuroml still goes via the neuroml object and the cell_morphology/morphology
accessors return an optional loaded_morphology.

⚠️ Breaking ⚠️

• Removed the load_*_raw functions, as loaded_morphology bundles the segment tree.
• The interface of load_* now has an extra indirection.
• The interface of neuroml changed.
• label_dict::import is now called extend for future safety (=C++ modules) and Python; calling things import (=a keyword) is not well received by black et al.

Issues

Closes #1981

[boeschf]

copy of *this intended when returning?

[thorstenhater]

no, good catch.

[boeschf]

looks good!