Surrogate: Multiple savepts

[teubert]

Requirement Text
Train surrogate model with different multiples of save_pts,

Background Information
Surrogate model currently generates matrices at one step size defined by save_freq. This works fine, but additional efficiency can be gained by using the largest step size possible with each step (e.g., however long you're at a single load.

Suggested Solution
Save multiple matrices with different step sizes and switch between them when simulating forward

DoD

• Implement feature
• Add tests
• Add to change notes for next release