Speed up `nest` and `unnest` for massive datasets in `tidySingleCellExperiment`

[stemangiola]

There is an example in the tidyseurat repository of how we can speed out nesting for very simple use cases using the split and merge functionality of Surat

[stemangiola]

@LiNk-NY this is one of the challenges I was mentioning.

[william-hutchison]

Hello, I would be happy to have a go at solving this.

[william-hutchison]

I have been able to considerably speed up nesting of a single column by using SingleCellExperiment's [] subsetting, but find my nested objects don't always cleanly unnest.

I will keep working at this, but if anyone else would like to have a go feel free! The changes I have made are available here for reference: https://github.com/william-hutchison/tidySingleCellExperiment/tree/master.