Add splice junctions input to STAR align

[cguyomar]

This PR adds splice junction files as an input for STAR align. This is necessary to perform 2 pass mapping (See STAR manual section 8.1), and useful for RNAseq variant calling (could be used in nf-core/rnavar I guess). I also added a small test case. The PR for the test data needs to be merged for it to work though. This is my first PR on nf-core, many thanks with your help :)

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