Draft: Lanson&Vila (2008) timestep criterion for Gizmo

This contains the original attempt to implement a better timestep criterion for Gizmo, based on Lanson&Vila (2008).

Needs further testing and refinement.

requested review from @mivkov

assigned to @bvandenbroucke

Just to keep track of things here as well: I'm not sure this is correct. They use a different notation in the paper, so it needs to be checked carefully.

See also Hietel, D., Junk, M., Kuhnert, J., Tiwari, S. (2005). Meshless Methods for Conservation Laws. In: Warnecke, G. (eds) Analysis and Numerics for Conservation Laws. Springer, Berlin, Heidelberg . https://doi.org/10.1007/3-540-27907-5_15