Adjust unsafe blocks for the BaseMatrixMut trait

[peamaeq]

In this trait you use the unsafe keyword for many safe expressions. However, I found that only 4 functions are real unsafe operations (see the list below).

We need to mark unsafe operations more precisely using unsafe keyword. Keeping unsafe blocks small can bring many benefits. For example, when mistakes happen, we can locate any errors related to memory safety within an unsafe block. This is the balance between Safe and Unsafe Rust. The separation is designed to make using Safe Rust as ergonomic as possible, but requires extra effort and care when writing Unsafe Rust.
Real unsafe operation list:

1. the from_raw_parts_mut()\offset()\mem::transmute() function(these are unsafe functions from the standard library)
2. the row_unchecked() function(row_unchecked() is a native unsafe function)

Hope this PR can help you.
Best regards.
References
https://doc.rust-lang.org/nomicon/safe-unsafe-meaning.html
https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html