Add create_dataset convenience forms for dataspace

[mkitti]

This pull requests adds some convenience forms for create_dataset when specifying a dataspace. It allows the dimensions to specified as a single tuple or as Ints. For example,

    h5open(fn, "w") do f
        create_dataset(f, "test", Int, (128, 32))
        create_dataset(f, "test2", Float64, 128, 64)
    end

[musm]

Makes sense