v0.9.0

Release notes:

• Rename KmerVector to KmerArray, with K dimensions, with each dimension having size N, instead of being a vector of length N^K.
• Make KmerArray{N, K} a subtype of StaticArrays.StaticArray{NTuple{K, N}}
• Change indexing of KmerArray to be offset by 1. Linear indexing is done with indices 0 through N^K-1, and cartesian indexing is done on each axis with values ranging from 0 through N-1.
• Remove AbstractKmerArray types
• Remove KmerColumns, KmerRows. Columns or rows of a matrix can now simply be wrapped by KmerArray to count K-mers of sequences. E.g. count_kmers!.(KmerArray{4, 2}.(eachcol(zeros(4^2, n))), sequences)
• K-mers of arbitrary sequences can now be counted by simply defining the method axis_index(::KmerArray, ::eltype(sequence))