only open regions to assemble file if it exists. closes #5.
only open regions to assemble file if it exists. closes #5.

EichlerLab/pacbio_variant_caller,EichlerLab/pacbio_variant_caller,EichlerLab/pacbio_variant_caller,EichlerLab/pacbio_variant_caller,EichlerLab/pacbio_variant_caller