quasimap: allow >=1 reads file(s) as input

[martinghunt]

Allow input of more than one reads file. Most common use case is user has 2 FASTQ files to use as input.

[ffranr]

This feature has been implemented with commit: b6a07409f05fa5cbfc33f12192574da09b6e36ee

Examples: 1) gramtools quasimap --gram-directory ./gram --reads ./reads --reads ./reads_2 2) gramtools quasimap --gram-directory ./gram --reads=./reads --reads=./reads_2 3) gramtools quasimap --gram-directory ./gram --reads=./reads --reads=./reads_2 --reads=./reads_3