What does this implement/fix?

It is common in fNIRS to have source and detectors in a criss-cross pattern. I try and illustrate this below (excuse the poor ascii art)...

Here sources are indicated by [s], and detectors by[d]. Channels are then formed between the sources and detectors indicated by a -, and by x where two sources and detectors have overlapping channels. For example there is a channel between s2 and d3, and one between s3 and d2 that overlap.

[s2]  -  [d2]  -  [s4] 
      x        x         
[s3]  -  [d3]  -  [s5] 

This approach is used purposely to reduce noise (the overlapping channels should be measuring the same brain region), and for redundancy (its common to have a source or detector that fails or is blocked by hair/scars/etc).

Currently if you have this criss-cross pattern then the location of the channels is very close and MNE throws an error similar to... The following electrodes have overlapping positions, which causes problems during visualization: S2_D3 hbo, S3_D2 hbo, ...

In the future I would like to develop a way to combine overlapping channels in to a single optimal channel. But for now I would just like to be able to force MNE to allow me to view the data as a topomap. So I have exposed the ignore_overlap variable that was available in the lower level topomap function.

Additional information

I am keen to hear other approaches to how I can address this issue. I probably should have opened an issue and discussed the approach before I coded it.

None of the current nirs test data have overlapping channels. I will make sure the next set I upload does.