Add option to pre-process CATMAID neuron before nblasting

[schlegelp]

Possible options:

• pruning of small branches
• Strahler index?

[jefferis]

See natverse/elmr#19

[schlegelp]

Would be straight forward to run these commands from via the R bindings, I guess.
How do you pass the processed neuron to nblast then?

[schlegelp]

Easiest way would probably to add an option to preprocess to the nblast_fafb package?

[jefferis]

The easiest would be to allow nblast_fafb to have extra arg(s) to run some additional processing functions.

[jefferis]

Snap.

[jefferis]

For info, if you look at that issue, you'll see that there are a couple of things that need some sigificant speedups (nat::resample and nat::strahler_order). I've done nat::strahler_order but resample is a bit more fiddly.