Change probability to ndf in Vertex

[jmcarcell]

As discussed in https://github.com/key4hep/EDM4hep/issues/318.

Looks like this is unused except in the converter.

BEGINRELEASENOTES

• Change probability to ndf in Vertex

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Changes in other repos:

• https://github.com/key4hep/k4EDM4hep2LcioConv/pull/71
• https://github.com/HEP-FCC/FCCAnalyses/pull/377

[jmcarcell]

I can have a look