Make multithreaded

[lukaas33]

I am not sure if this tool is still under active development, but one feature that would be desirable is multihtreading.

I have looked at the Python helper script and see that it reads all SNPs and indels into memory, annotates them and only later writes them to a file. This would allow for some batching and running bam-readcount in parallel in between.

This would potentially greatly speed up this tool, which is now running quite slowly.