DSL2: Add lane attribute to BAM inputs

[TCLamnidis]

When users provide multiple BAM files from the same library as BAM input for nf-core/eager, the only attribute that can be different between rows (other than the BAM path) is the Lane number. Therefore, the lane should be propagated into the meta map of the BAM input subchannel.

Adding this is easy enough, but the following will need to be ensured:

• Adding this attribute does not break any downstream channel manipulations like .join, .combine etc.
• BAMs across lanes are merged before Deduplication (as should be the case with FastQs), at which point the lane attribute will be dropped from the channel.
• Lane info should also be in the output name for BAM filtering output (or BAM filtering occurs after merging at library level).