Allow users to define memory usage in 'samtools sort'

[ycl6]

This PR makes the following changes:

• When using CPU/juicer.sh and CPU/mega_from_bams_diploid.sh, users can define memory per thread in samtools sort using --mem, for example, --mem 8G
• For PBS and Slurm schedulers, the memory per thread setting in samtools sort is calculated based on the preexisting resource requests

With the ability to increase the memory per thread setting, it will reduce the number of temporary files produced during sorting. With large dataset, users can set a larger value to prevent the "Too many open files" error from happening (e.g. #343), as the default used by samtools sort is only 768M per thread.