This came up in #6613.

We document the value beta of the entropy viscosity stabilization with an optimal value of 0.056 in 2D and 0.078 in 3D as determined in the first ASPECT paper (Kronbichler et al 2012). However, we set the default value to 0.056 and do not modify it for 3D models. Therefore, unless users set it to 0.078 manually in 3D models we are actually using a too small value in 3D models, which may lead to too little stabilization.

we should instead read in beta per spatial dimension and set the default to 0.026. Them multiply it with dim in the code after reading in the value.