Fixing vectorisation of sigma mu adjustment

[mmpagani]

[micheles]

You are effectively disabling the vectorization, i.e. there is an interpolation for each magnitude, even if the magnitudes are all equal: this is no good performance-wise. I need a test exhibiting the issue and then I can fix it properly.

[micheles]

I was able to see the issue when using a ModifiableGMPE.