colouring ngscene objects

[jefferis]

more advanced addition to #55

• colour_segments How should this work? Need to associate segments with a colour / function. Would be nice if we could do this by nblast results easily

[jefferis]

I think the simplest thing that we need is a function that takes a vector of colours in the same order as the segments returned by ngl_segments() and inserts a named list into the neuroglancer scene object. ngl_encode_url should take care of encoding the R list back into json so there is no need to modify the json directly.

To get some ideas for what nblast results (which is one way we might want to colour neurons), see http://natverse.org/nat.nblast/articles/NBLAST-Clustering.html including the referenced colour_clusters function.

[jefferis]

@SiqiFang some notes

[jefferis]

implemented in #91. Could still do with an even simpler way to combine multiple scenes in order to merge their segments/colour schemes.

[jefferis]

Thanks @SiqiFang @SridharJagannathan