Magniture-Area scaling relationship to mimic point ruptures

This pr introduces a new scaling relationship `PointMSR` which can be used to mimic point ruptures. This MSR returns, for whatever magnitude value, a very small value of area (1e-4 km**2, corresponding to a 10 by 10 meters square).

I also updated the peer tests for area source to make use of the PointMSR. Indeed the Peer reference solutions are calibrated for point rupture. Until now we were able to make the tests passing only because we were using a large discretization step for discretizing the source (30 km). By reducing it to 10 km, the PeerMSR do not allow to fit the data, and therefore we need to use the PointMSR. Because of this reduction in the area source discretization step, the time associated to the tests execution increased a bit (from ~130s to ~150s on my laptop)