Less opinionated splitting

[ceesem]

this fixes a number of small issues that have cropped up over time.

• Removes pyembree from being imported unless it's actually used
• Fixes a logical flaw in axon/dendrite splitting that assumed axons were always downstream of root. Now the axon of a split is the side that has the highest fraction of outputs compared to all synapses.
• Remove deprecated Numpy dtypes
• Remove numba dependency which offered modest speedups at the cost of a lot of depenencies
• Fix a bug allowing non-inclusive 'downstream_nodes' call on a skeleton.

[ceesem]

Also, I am exposing the synapse_betweenness function which was previously hidden for some reason.