Fix profile extraction: stop merging volume data, initialise all ProfileData fields

[Copilot]

Profile processing should take volume datasets as a named tuple and not merge them into one grid first, which is slow when there are many volume datasets. It also needs to pass topography data through and keep ProfileData.TopoData usable.

Most of this was already in place (TopoData field, named-tuple create_profile_volume!, topo arguments on extract_ProfileData!). Two gaps remained:

• ProfileData constructor

  • new(...) was given 7 values for a 9-field struct, so TopoData, PointData and ScreenshotData were left undefined on a fresh profile.
  • It now initialises all 9 fields to nothing.
  • Without this, reading profile.TopoData or show(profile) fails on an unset field.

• extract_ProfileData(file, n, datasetfile) convenience wrapper

  • It no longer calls combine_vol_data.
  • It passes the NamedTuple of volume datasets straight to extract_ProfileData!, so each dataset is cross-sectioned on its own.
  • It now also forwards TopoData and ScreenshotData, which load_GMG already returned but the wrapper dropped.

# before: merge every volume dataset onto one grid, then slice
extract_ProfileData!(profile, combine_vol_data(VolData), SurfData, PointData; ...)

# after: slice each volume dataset directly, and include topography
extract_ProfileData!(profile, VolData, SurfData, PointData; TopoData = TopoData, ScreenshotData = ScreenshotData, ...)

Notes for review

• The package could not be loaded in my environment (dependencies not installed), so none of this has been run. The test file also downloads its datasets over the network.
• The new test only checks that TopoData is empty on a fresh ProfileData. Topography extraction itself is not covered.
• Some existing calls in test/test_ProfileProcessing.jl may not match any current method. They pass nothing as the volume data, and pass screenshot data as a fifth positional argument. I left them unchanged.

[mthielma]

Copilot went a bit too far here, as it started to fix all kinds of issues, not only the one I asked for. I am therefore also closing this one.