probe_eddy_current: implement surface scannng

[Arksine]

This PR build on #6536, adding support for "surface scanning". The first commit resolves a minor issue where PROBE_EDDY_CURRENT_CALIBRATE removes all previous autosaved configuration, including the reg_drive_current option saved by LDC_CALIBRATE. If necessary this commit can be removed from this PR and added to another (or to the original).

The rest of this PR makes the necessary modifications to probe_eddy_current.py, probe.py, and bed_mesh.py to support scanning a surface using an eddy probe. In addition, a graph-mesh.py script has been added to visualize and analyze mesh state, which can be dumped to a json file using the new BED_MESH_DUMP command.

[KevinOConnor]

Thanks. I added your fix to PR 6536 and rebased that branch.

Separately, I'll roughly plan on merging PR 6536 into the main repo first, and then look to merge this afterwards. Let me know if you have any concerns with that.

Cheers,
-Kevin

[Arksine]

Sounds good to me. I rebased this against your changes for now and can rebase against master before merging if necessary.

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[Arksine]

Hi Kevin, it looks like GitHub automatically closed this PR since I created it against the work-ldc branch. I went ahead and created a new PR rebased against the latest master. Thanks.

[KevinOConnor]

Oh - sorry - I did not realize github would do that. Thanks for creating the new PR.

-Kevin