Salvus 0.12.13

When building meshes with exterior domains for modeling the gravitational potential, all material parameters will automatically be set to zero. Furthermore, the exterior is assigned a unique layer id, which increments the layer id at the surface by one.

Fix a bug in the visualization widget of an inverse problem, which could cause a ZeroDivisionError if there were one or more events without windows in the data selection.

Fix a bug in the window picking algorithm which caused a TypeError for constant traces.