init_3_timelevels leads to problems with global interpolation

The former test case for CarpetInterp "waveinterp-2p.par" crashes with an assertion failure because there is an unintended "extrapolation in time". This parameter file uses "init_3_timelevels"; if this parameter is not used, then everything is fine.

We need to check whether the extrapolation in time occurs while the three-timelevel-initialization algorithm is running, where this may be fine, or whether there is an error in the way the past time levels are set up when init_3_timelevels is used.

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