disable writing to `asnumpy()` references

[garrettwrong]

There's an interesting point here, though: we have to call copy on the ndarrays returned by asnumpy to avoid modifying the original Image? We may want to consider forcing asnumpy to make a copy (or to mark it read-only) to avoid this issue. This may be less important to Images, but when they're part of a source, we certainly don't want people to go around modifying them by accident (especially if the sources are to be seen as immutable).

Originally posted by @janden in #884 (review)