refactor: use struct with matrix operations instead of raw array

[carrascomj]

Motivation

In order to use more chained matrix transformations, a better API should be implemented to apply matrix transformations and element wise operations.

Implementation

Change kernel from an alias type to a proper struct with matrix operations.

Progress

• working locally.
• tests implemented.
• implement also Mul for Kernel -> Kernel (not needed for the current transformations).