Fallible methods for decomposition

[ThierryBerger]

• improves #264 ; but the assert is still here.
• Fix #347

This approach is to wrap computation results into Results when it makes sense.

We can probably go further into this approach, and really hunt for any misconfiguration and return an error whenever the inputs don't make sense, but let's discuss this first.

I considered adding try variants, but it doesn't add too much value so I went straight to migrating to fallible everywhere.