Functions for manipulating the allele matrix

[magnusdv]

These would be useful to have:

• getAlleles(x, ids, markers)
• setAlleles(x, ids, markers, alleles)

For instance, to remove all genotypes of individuals "a" should be done by

x = setAlleles(x, ids = "a", alleles = 0)

Alternative function names: getAlleleMatrix, setAlleleMatrix.

[magnusdv]

Implemented in a85cbff.
(Forgot to close issue).