Display warning to user when k-mer length is below specific threshold

[glabbe]

There is currently no lower limit to k-mer length, and this could cause issues if the k-mer length is, say, below 6-10 bases, and the k-mer is found hundreds or thousands of times in a genome assembly. The user should be warned when the k-mer length is too short for the sequence to have specificity for a particular genome region.