Support CSI files too

[muffato]

This pull-request is inspired by our discussion this morning on the problems with -resume you've faced when testing these changes for #171.

ch_bai is meant to be the channel of the BAM index that is then used as an output of the sub-worfkflow (in emit:). The same variable name should be used when initialising it to an empty channel, getting the output of SAMTOOLS_REINDEX, and putting into ch_data_for_stats.
It's good to add support for CSI, but the way the nf-core samtools module works, they're in different output channels and mutually exclusive. Rather than changing to just CSI, I propose to simply mix the two channels. So that ch_bai remains the one variable we use, and it will have whatever index was created by samtools.

I don't know if this change would have fixed your issue with -resume, I haven't tested that.

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[sanger-tolsoft]

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