Allow for snpEff db without versions

[maxulysse]

Description of feature

Currently in Sarek we suppose that the database is build following the format genome.db, like GRCh37.75, GRCh38.105, GRCh38.99, and so on...

But actually there are lots of databases with only genome like hg19, hg19kg, hg38, hg38kg, testHg19ChrM for Homo_Sapiens.

Or for other organisms, only the former could be available, such as Oryza_sativa