Improve combine_scorefiles

[smlmbrt]

I agree, we should consider how to make this faster. Potential solutions (thinking out loud):

• Not appending to a file, writing to an open file connection
• Processing multiple files in parallel (multiple threads)

Also related to #64

Discussed in #110

^{Originally posted by mglev1n June 15, 2023}
When using pgsc_calc for more than a few scores, the pipeline seems to take a very long time. Despite using the --parallel argument and having access to HPC resources, the step that combines scores into a single scoring file seems to run serially (reading score1, performing QC, appending to scoring file; reading score2, performing QC, appending, ...). Is there a more efficient approach for creating a combined scoring file (either using the pgsc_calc pipeline, or preparing the combined scoring file prior to running the pipeline)?