Separate linear operator from matrix trait

For context, this idea is motivated by vbarielle/sprs#118.

The matrix trait is rather large at the moment:

```rust
pub trait Matrix: Sized + Clone + Mul {
type Field: Field;
type Row: FiniteDimVectorSpace;
type Column: FiniteDimVectorSpace;
type Transpose: Matrix;
fn nrows(&self) -> usize;
fn ncolumns(&self) -> usize;
fn row(&self, i: usize) -> Self::Row;
fn column(&self, i: usize) -> Self::Column;
unsafe fn get_unchecked(&self, i: usize, j: usize) -> Self::Field;
fn transpose(&self) -> Self::Transpose;

fn get(&self, i: usize, j: usize) -> Self::Field { ... }
}
```

For many algorithms, access to the internal structure of the matrix is not necessary. It also does put some burden on trait implementors that may be possible to avoid.

A suggested solution would be to introduce a more abstract linear operator inspired by SciPy's [interface with the same name](https://docs.scipy.org/doc/scipy-1.0.0/reference/generated/scipy.sparse.linalg.LinearOperator.html) as a supertrait.

```rust
pub trait LinearOperator: Sized + Clone + Mul {
type Field: Field;
type Row: FiniteDimVectorSpace;
type Column: FiniteDimVectorSpace;
type Transpose: LinearOperator;
fn nrows(&self) -> usize;
fn ncolumns(&self) -> usize;
fn transpose(&self) -> Self::Transpose;
}

pub trait Matrix: LinearOperator {
fn row(&self, i: usize) -> Self::Row;
fn column(&self, i: usize) -> Self::Column;
unsafe fn get_unchecked(&self, i: usize, j: usize) -> Self::Field;
fn get(&self, i: usize, j: usize) -> Self::Field { ... }
}
```

It would be useful to include matrix multiplication there as well, but I have a feeling that this is hard to express in today's Rust. It also seems like a separate issue as the original matrix trait does not include this either.