`solve_mut` inconsistent naming

The following methods take a mutable reference to a matrix and store the result directly into the (mutable) input matrix:

```
nalgebra::linalg::Cholesky::solve_mut
nalgebra::linalg::FullPivLU::solve_mut
nalgebra::linalg::LU::solve_mut
nalgebra::linalg::QR::solve_mut
```

However, this naming is inconsistent with the naming conventions used for the `mut` postfix in other parts of the library, as well as the Rust standard library, where `mut` signifies that `self` is mutated, not the input.

I propose renaming these methods to `solve_in_place`, or my personal favorite `solve_into` (meaning that we solve "into" the provided storage). "in place" feels somewhat more ambiguous as it might also sound like it's in-place with respect to the decomposition.