Public interface for submatrices and minors

Would it be possible to expose public interfaces for [submatrices](https://en.wikipedia.org/wiki/Matrix_(mathematics)#Submatrix) and [minors](https://en.wikipedia.org/wiki/Minor_(linear_algebra))?

Could it also simplify some internal logic? For example:

https://github.com/dimforge/nalgebra/blob/1576a1517ad652db5bfa1bb9071eeadc699b4328/src/linalg/inverse.rs#L71-L74

https://github.com/dimforge/nalgebra/blob/1576a1517ad652db5bfa1bb9071eeadc699b4328/src/linalg/determinant.rs#L38-L41

https://github.com/dimforge/nalgebra/blob/1576a1517ad652db5bfa1bb9071eeadc699b4328/src/base/matrix.rs#L2111-L2113