Added less stringent dependency on pysam. Previous dependency requeirment could cause error.

Merge branch 'hotfix/pysam_dependency' into develop
fixed pysam dependency bug.
Merge branch 'master' into develop
Added catch of exeption when no reads are mapped on contigs. More informative error message is given.
Added more informative error message when no reads are mapped to contigs.
Merge branch 'hotfix/no_mapped_reads_error'