Bugfix: convert keys to list

[ChristophH]

Fixes "AttributeError: 'dict_keys' object has no attribute 'index'" that occurred when using iGenomes GRCh38 Line 92 already wraps exon.keys() in list() - this seems to have been forgotten in line 199.

nf-core/slamseq pull request

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[lpantano]

Hi, I am closing this PR since is too old. Feel free to reopen with updated code to get the tests passing. Sorry we didn't get to merge on time.