Change assignments regarding linear system of eq and least squares

[palmoreck]

Change exercises regarding module creation of:

• solve_linear_system_of_equations.py in 1_ecuaciones_lineales.ipynb
• utils.py in 3_minimos_cuadrados.pynb

as their implementations are almost equal. Maybe consider to implement forward, backward substitutions instead of using scipy.linalg.solve_triangular

[palmoreck]

"almost equal" -> what a person implements