should handle case of no isolated spikes found in a batch

[magland]

This is related to the following issues #525 #335

It seems that kilosort3 crashes if no isolated spikes are found in a batch during datashift2.m

Sometimes a dataset simply contains no spikes, in which case I would expect kilosort to return an empty sorting rather than to crash. This can happen, for example, when sorting a subset of channels that do not have signal.

This crash also occurs if there are no spikes within a sub-time-window of the recording corresponding to a kilosort batch. This can happen when there is a dead zone for part of the dataset, for whatever reason.

The error occurs at this line because size(dd,2) is zero:

Kilosort/mainLoop/extractTemplatesfromSnippets.m

Line 56 in d55179f

wTEMP = dd(:, round(linspace(1, size(dd,2), nPCs)));

The error message is Subscript indices must either be real positive integers or logicals.

LMK if you want me to work on a PR to gracefully handle this case.