Fix running averages for restarted simulations

[TobiKattmann]

Hi,

When restarted at e.g. Time-Iter 2 the first value would be divided by 3 (number of timesteps counting from zero).
So if your first value in that restarted sim is a 3, the running average would be 1.

This is fixed with this commit by setting WINDOW_START_ITER = RESTART_ITER in the Config Postprocessing,
if the option is not set. If the option is set, but incorrectly, an error is raised.

Related Work

Original implementation #836
Change in previous behavior introduced in #1548

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[TobiKattmann]

Thanks for the review :) Once the tests pass with this tiny commited regression value change for the hybrid case - I'll merge this in

[pcarruscag]

Sounds good thanks for the fix.