Missing methods for converting between NativeLabels and Indices

As outlined by @oxinabox in #17

- [ ] It should be possible to convert upwards in the number of labels. at least for integers. (e.g. from a `Indices{_,2}` to a `NativeLabels{_,3}`)

```julia
enc = labelenc(["copper", "tin", "gold"])
convertlabel(enc, [2, 1])
```

- [ ] We should support single integers when the target encoding is unambigous

```julia
convertlabel(enc, 3)
```

- [ ] It would be useful to have the source encoding inferrable from just the type. I do think to remember though that there was a good reason why I didn't do it in the first place

```julia
convertlabel(enc, 3, LabelEnc.Indices)
```