Allow for preindexed custom genomes in dsl2

[TheodoreMarkulin]

DSL2 does not allow for using already indexed custom genome

I use a custom variation of GRCh38 that includes control sequences. It takes a while to index the entire genome using BWAMETH_INDEX and in practice this should only need to be done once for each custom reference.

It appears that the current dsl2 branch only considers preindexed igenomes references: https://github.com/nf-core/methylseq/blob/acdc4afccf9fd3dd4150fc4963134300cd5d26d7/subworkflows/local/bwameth.nf#L25-L29

The current branch does this when the --bwa_meth_index parameter is passes:

https://github.com/nf-core/methylseq/blob/03972a686bedeb2920803cd575f4d671e9135af0/main.nf#L77-L94

Moving the logic over from the current master branch to the dsl branch would help when doing work with custom genomes.