Add option to write unassigned reads

[tmaklin]

It is currently possible for some input reads to remain unassigned. mGEMS should have an option (off by default) to write these reads to a separate bin.

[tmaklin]

Implemented in https://github.com/PROBIC/mGEMS/commit/21ca63bfefe39335a89b31a62e9b3be40d15c54e, https://github.com/PROBIC/mGEMS/commit/9c8b6dc3713fe7a3e2ae8ef2dac27edef9c415d9, and https://github.com/PROBIC/mGEMS/commit/0c71f667b5ceafcecb1dbc40c34a5f9be7a09848.

Added the --write-unassigned toggle, which causes the

• mGEMS bin program to write out a bin named unassigned_reads.bin, containing the ids of the reads that pseudoaligned to a reference sequence but were not assigned to any group.
• mGEMS program to write out the fastq files for these reads in files prefixed with unassigned_reads.

mGEMS extract can be used to extract the unassigned reads by supplying the unassigned_reads.bin file as the --bins argument, similarly to any other bin.