Conversion to dotprops for nblast(neuron, dotprops) [or vice versa] is rather inefficient

[jdmanton]

As the conversion happens in WeightedNNBasedLinesetMatching(), this means that the objects are converted for every comparison, not just once at the start, making everything much slower than it needs to be.

[jefferis]

There is a difference between the conversion in WeightedNNBasedLinesetMatching and converting using e.g. dotprops(,resample=T). In the first case the direction vector is the vector from each point to its head point, without resampling. In the second case it is based on k nearest neighbours. It would probably be best to stick to pre-conversion to dotprops in most cases. I haven't profiled in ages, but I am not sure that the conversion part of WeightedNNBasedLinesetMatching.neuron is that slow since there is no nearest neighbour calculation or eigenvector decomposition.

[jdmanton]

I meant the conversion I added in d55c544 to allow it to handle the situation where query and target are not of the same class. I tried doing this higher up, in nblast(), but this messed a few things up — it looks like it is worth me retrying this and attempting to fix the problems.