[FEAT] Submatrix as a refrence

[KjetilIN]

🚀 Feature: <>

Description
Currently there is no clear use-case for retrieving a sub matrix as reference. In case making a new matrix costs too much.

Proposed Solution
Current function signature

    /// Not Implemented
    pub fn submatrix_as_slice(&self, rows:Range<usize>, cols:Range<usize>) -> Result<Matrix, MatrixError>{
        //TODO: Ignoring for now, not use case clear 
        unimplemented!()
    }