Added option for setting unmapped BAM record

[FelixMoelder]

This PR makes the cigar parameter of a BAM record optional when setting variable length data.
Doing so allows to create unmapped records.

[coveralls]

Pull Request Test Coverage Report for Build 630

• 13 of 15 (86.67%) changed or added relevant lines in 3 files are covered.
• 23 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.1%) to 60.067%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/bam/record.rs	8	10	80.0%

Files with Coverage Reduction	New Missed Lines	%
src/bcf/mod.rs	1	86.06%
src/bcf/header.rs	22	73.66%

Totals
Change from base Build 621:	0.1%
Covered Lines:	21406
Relevant Lines:	35637

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💛 - Coveralls

[johanneskoester]

Perfect!