added less stringend dependency on pysam

Merge branch 'hotfix/pysam_dependency'
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' into develop