allow for the user to pass f_ref and eps

[u-dupletsa]

• allow user to pass f_ref (reference value is 50Hz as it was before)
• allow user to pass derivative step; distinguish between eps and eps_mass, as mass parameters require a smaller step since they are precisely constrained
• modified tutorial to include a note on the possibility to set the reference frequency and change f_ref value; added a small paragraph (at the end of the tutorial) about numerical derivative in gwfish and the size of eps

[jacopok]

Looks good to me!