flush stderr/stdout from background processx to avoid blocking

[ivokwee]

Sometimes the background process seems to halt or blocked. I suspect blocking of stderr/stdout is happening as described here https://processx.r-lib.org/

Note, that the connections have a buffer, which can fill up, if R does not read out the output, and then the process will stop, until R reads the connection and the buffer is freed.
Always make sure that you read out the standard output and/or error of the pipes, otherwise the background process will stop running!

So during process checking we need to flush the stderr/stdout buffer to avoid any fill up. I changed the code.

[mauromiguelm]

• tested on wsl, did not block the app as before.
• added a small comment about computation of multiple datasets
• recommend storing the process directly on lists, and not on intermediate variable, as I tried this solution before and it did not work for multiple processess.