Support setting the compression level for bam::Writer

[juliangehring]

Allow setting the compression level when writing BAM/CRAM files. Adds

• Writer.set_compression_level()
• CompressionLevel enum to specify the desired compression level
• CompressionLevelError error enum
• test cases

[coveralls]

Pull Request Test Coverage Report for Build 644

• 49 of 55 (89.09%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 60.163%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/bam/mod.rs	49	55	89.09%

Totals
Change from base Build 643:	0.1%
Covered Lines:	21474
Relevant Lines:	35693

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💛 - Coveralls

[juliangehring]

Should resolve #142.
@wwood, what do you think?

[wwood]

Sure, looks good, for what my opinion is worth. This is enough for my use case, though I thought you were planning bcf too?

[johanneskoester]

Great addition, thank you!