mat64: reverse signatures for decomposition matrix extractions

[kortschak]

Also provides some sugar to ease this.

@btracey @vladimir-ch Please take a look.

Closes #435.

[btracey]

Why the "To" endings? It seems like L (for instance) is just as clear.

[kortschak]

There are two usage syntaxes, dst = x.PTo(nil) and x.PTo(dst). The first is obviously an assignment to dst, but the second would be less obvious in the case that it were written x.P(y). The To suffix clears that use intention (at a minor cost in the first case where To(nil) is a little upsetting, but I think the trade-off is worth it (and could be apologised). It lends its origin from io.WriterTo.

[kortschak]

I'll wait for @btracey to make sure he's happy with my argument above.

[btracey]

Thinking about it, I think the related argument is that usually the receiver is modified. This is a difference, so that difference should be clear.

[btracey]

LGTM

[btracey]

This is a pretty big breaking change, at least for code I've written. Is it possible we could merge repos into gonum/gonum and then make this change? It sounds like there's nothing in the way of migration that will change.

[kortschak]

I'm happy to wait. This is relatively small and I'll just copy past over to the new location when we are ready if that's what you want.

[kortschak]

Curious though. How does delaying the merge of this help?

[btracey]

My suggestion was that this is a (decent-sized) breaking change, and it might be nice to delay that until the merge. This way, old code can remain working without update.

[kortschak]

Done in gonum/gonum.