ENH: Allele mapping

[alimanfoo]

Proposed to create functions to map genotypes, haplotypes and allele counts arrays between different allele indexing schemes.

Implementation plan

• Add create_allele_mapping(from_alleles, to_alleles, dtype) public API function to create an allele index mapping given two sets of alleles.
• Add genotypes_map_alleles(gt, mapping) public API function to transform a genotypes array using an allele mapping.
• Add haplotypes_map_alleles(ht, mapping) public API function to transform a haplotypes array using an allele mapping.
• Add allele_counts_map_alleles(ac, mapping) public API function to transform an allele counts array (either 2D or 3D) using an allele mapping.
• Add numpy and dask backends.
• Add unit tests.

Notes

This is a porting of functionality from scikit-allel version 1, see create_allele_mapping, haplotype_array_map_alleles, allele_counts_array_map_alleles.

Add tests to ensure map_alleles works with either fixed length string dtype or object dtype.

Use numba for numpy backend implementations of the ..._map_alleles functions.