Speed-up CoregPipeline fit function

[adehecq]

For speed up, this line should not be called on the last iteration, since tba_dem_mod is not used afterwards. https://github.com/GlacioHack/xdem/blob/2564f5f1c4654f1b153a88193851f40a55f09226/xdem/coreg.py#L1140