Record readback x,y,z values of panda gridscans in nexus

[DominicOram]

Following the panda gridscans we will end up with an hdf file of motor positions. We should put these into the nexus file for downstream applications to potentially use.

Following discussion on https://diamondlightsource.slack.com/archives/G017HTC55M1/p1729861842852669 the following format was decided upon:

• The current /entry/sample/transformations/sam_{x,y,z} are changed to instead contain readback values as VDS links from the panda hdf files
• The angle transforms are kept the same as we do not record readbacks for these
• The depends_on chain for sam_{x,y,z} is unchanged (which means sam_z depends on omega, which is actually still setpoints but that's fine)
• We add in a new transformation for each of sam_{x,y,z}_set next to the readbacks that holds what we were previously writing
• The new transformations will have the same attributes except they will follow a depends_on hierarchy based on {x,y,z}_sets (except sam_z as this will still depend on omega, which will have setpoints in)
• We add a variant (https://manual.nexusformat.org/datarules.html#variants) on sam{x,y,z} linking to @variant=sam{x,y,z}_set

Note that the data coming out of the panda may currently be in encoder counts but we want it in um. Ideally this conversion should be done in the panda when it is writing the data out

Acceptance Criteria

• Panda readback data is put into the nexus file in the above format
• There are tests on this