Add option to specify the minimal contig length used for the pyDamage analysis

[alexhbnr]

Description of feature

Currently, all contigs that were assembled by an assembler will be passed on to pyDamage when the --ancient_dna workflow is activated. However, from simulations we know that for very short contigs we will almost never have sufficient data to enable us to make an accurate prediction of whether a contig was ancient or not. Therefore, it would be beneficial to have an option to specify the minimal contig length for the pyDamage analysis.

@maxibor is currently implementing this feature into pyDamage so once it's available the aforementioned option could be added.