Implement a more secure way to store the domain mapping of curved meshes

The current way of storing the domain mapping is to save the code of the definition of the mapping as a string, and load the mapping after a restart using `eval`.
This is a security issue, since malicious code in a HDF5 file can be executed.

We should use a different approach to store the mapping, for example storing an interpolation (of a higher degree than the polydeg of the solver, such that a simulation can be restarted with a higher polydeg), and showing a warning to the user if a simulation gets restarted with a higher polydeg than the degree of the interpolation of the geometry.