v4.0.0: Axon carrying dendrite

This major release allows for statistics on mixed subtrees; for instance Axon Carrying Dendrites.
To support this, many changes were made, including:

• Morphology class accepts only morphio objects, not files anymore. (#1120)
• Replace iter_* methods by properties in core objects and improve iter_segments. (#1054)
• NeuriteType extended to allow mixed type declarations as tuple of ints. (#1071)
• All features return built-in types (#1064)
• Morphology class also allows mutable morphio objects to be passed explicitly. (#1049)
• Morphology class uses morphio immutable class by composition, istead of inheritance. (#979)
• Morphology level radial distance features use the somata as reference point. (#1030)
• Make neurom.core.Population resolve paths. Symlinks are not resolved. (#1047)
• Mixed subtree processing can be used in morph_stats app via the use_subtrees flag. (#1034)
• neurom.view.[plot_tree|plot_tree3d|plot_soma|plot_soma3D] were hidden from the
  neurom.view module. They can still be imported from neurom.view.matplotlib_impl. (#1032)
• Mixed subtree processing. (#981)
• Deprecated modules and classes were removed. (#1026)

Please note that this will require updates to software using NeuroM as a library.
There is a migration guide here: https://neurom.readthedocs.io/en/stable/migration.html
In addition, one can bound NeuroM's version to be <4 before migrating.
Note, however, that all development will be on the "v4" code base going forward.