rename integration -> discretization where appropriate

[kenyastyle]

This will be unpopular and other ViennaTools repos will need to be updated as well but I strongly believe we need this renaming for clarity. I have not found any reference in math literature where the schemes from the former IntegrationSchemeEnum are called "integration schemes" because they do no integration. It is part of the reason I was confused about the topic.

It also helps since we already have two time integration methods and possibly more will come.

What do you think?

[FilipovicLado]

Thanks for the naming cleanup. You are right that IntegrationScheme is not the best name. However, DiscretizationScheme is still too generic here, since it doesn't distinguish between time and space discretization.

A clearer name might be SpatialScheme. This keeps the naming precise while avoiding long identifiers like SpatialDiscretizationScheme.

I’d suggest renaming to SpatialScheme instead of DiscretizationScheme.

[tobre1]

I agree this would be a better name.
However, IntegrationScheme is likely already used in some simulation workflows. I suggest renaming it everywhere possible, but keeping IntegrationScheme as an alias and marking it as deprecated in the documentation. We can keep it for a few more releases and then remove it fully.