Allow multiple parameters for custom output function

[da-crivelli]

If a user wants to write their own reduction function (e.g. Galileo - Rankine criterion, or a custom safety factor output) they currently would need to use some post-processing in Paraview or other software.

In a future version this should be possible:

fea.set_outputs([{
            "output_var": ("s1","s2","s3"),
            "reduction_fun": myfun,
        }])

def myfun(s1, s2, s3):
    return something