Fix bug in loading of NBI data, NBI zero for all times

added 5 commits

• 861fb7a0...59f7cea7 - 4 commits from branch master
• 169b7130 - Fix bug in loading of NBI data, NBI zero for all times

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@osauter I just found a bug in the core_sources that appears for older discharges, e.g. #59151 where NBI1 was not used but gdat(59151,'powers') gives a non empty nbi1 field, but its all zeros. But its now fixed.

I also had a wrong usage of tgrid_nbi in the DNBI section, that is now fixed.

ok good

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