add a load and dump of field map test executable

For offline studies I am doing, in the past i was loading the Magnetic Field and use gaudipython to dump a root file to then pass forward in other tools. With dd4hep, and since U2 uses latest v8 maps , I tried to configure an app but i failed.

I didn't find an easy way to configure and run this with a configured dd4hep setup, so I ended up writing the dumper as part of Detector magnet tests.

In case, i leave this in draft as it might be useful for other studies such as compute the integrals of B-dl from dd4hep inputs directly and store tuples.

Currently MR is on top of lcg-105-compat, but can go to master as well.

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