Resume single cells export

[jccaicedo]

If the export of single cells stop for any reason, the user needs to run everything from scratch. Given that exporting single cells is time consuming, we can support a "resume" mode similar to the one we have in profiling.

We currently ask the user if they want to delete the previous directory, which made sense for sampling. Since we don't do sampling anymore, we can replace that functionality with resuming.