Explain why each use of `unsafe` is sound

[sebcrozet]

See that discussion on reddit. Doing this will reassure others, that our unsafe code is actually sound.

We should add comments explaining why each unsafe is sound throughout the nalgebra source code.

Doing this will take an extremely large amount of time so any incremental contribution is welcome. In other words, event adding one comment for one unsafe statement is a great contribution and justify a PR.

All use of mem::uninitialized can be ignored and left uncommented as we intend to get rid of them. See #556.

[danieldinu2030]

Hello, I tried providing good explanations for several files where unsafe is used. View PR #1501 for more details.