Response Matrices: unify approach

[amereghe]

Let's have a unique approach to the calculation of response matrices, i.e.:

• calls to LGEN files are under if statements;
• strengths are loaded in specific files called inside the for loop;
• a unique naming convention.

[amereghe]

This issue can be solved only after issue #15 is solved.

[amereghe]

Issue #15 solved by PR #18

[amereghe]

This issue is tightly bound to issue #33