Vcf-dedup

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* Minor changes
- Fixing bug when tie in AF and variant calling quality the selection of the "first" variant was not deterministic

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* Minor changes
- Fixing bug when the VCF contains no duplicates

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* Major changes
- Variants are kept in a buffer of size 10000 and only flushed to disk when full
- Sort return code is properly managed. When return code != 0 raises an error.

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* Major changes
- Added parameter `--mem-percentage` to control the amount of available memory used by sort command. This is quite low otherwise, default value now set to 80%.

* Minor changes
- Added parameter `--log-file` to write logs to a file. If not provided these are still written to the standard error.

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* Minor changes
- The sort temporary folder parameter is fixed

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* Minor changes
- The output of sort operation is now bgzipped to better process big VCFs

* Known issues
- The sort temporary folder parameter is broken