chore: upgrade z5 library to 3.0.1 (remove xtensor dependency)

[sameeul]

Summary

Upgrade z5 library from 2.0.20 (with xtensor dependency) to z5 3.0.1 (header-only, no xtensor).

This is a clean migration that eliminates 2 transitive dependencies while using the modern z5 3.0.1 API.

Key Changes

• Dependencies removed: xtensor (>=0.26,<0.27), xsimd (>=13,<14)
• Dependencies added: z5py >=3.0 (version pin in conda_cpp.txt)
• CMakeLists.txt updates:
  • Removed GCC 7.5 version check (z5 3.0.1 requires C++20)
  • Removed Boost dependency check (not needed for z5 3.0.1)
• Header file migration (omezarr.h, raw_omezarr.h):
  • Removed xtensor includes
  • Added z5 3.0.1 ArrayView API includes
  • Migrated data reading from xt::xarray to std::vector + ArrayView

API Migration

Before (z5 2.0.20 + xtensor):

#include "xtensor/containers/xarray.hpp"
#include "z5/multiarray/xtensor_access.hxx"

xt::xarray<FileType> array(shape);
z5::multiarray::readSubarray<FileType>(ds, array, offset.begin());

After (z5 3.0.1):

#include "z5/multiarray/array_view.hxx"
#include "z5/multiarray/array_access.hxx"

std::vector<FileType> buffer(data_height * data_width);
auto view = z5::multiarray::makeView(buffer.data(), shape);
z5::multiarray::readSubarray<FileType>(*ds, view, offset.begin());

Benefits

✓ Eliminates 2 transitive dependencies (xtensor, xsimd)
✓ Simpler conda environment configuration
✓ z5 3.0.1 headers come with z5py conda package
✓ Uses modern C++20 header-only library
✓ No functional changes to zarr reading behavior

Files Changed

• CMakeLists.txt: 12 lines removed (GCC 7.5 check, Boost check)
• ci-utils/envs/conda_cpp.txt: Added z5py >=3.0, removed xtensor/xsimd
• src/nyx/omezarr.h: Migrated to ArrayView API
• src/nyx/raw_omezarr.h: Migrated to ArrayView API

Testing Checklist

• CMake build succeeds with -DUSE_Z5=ON
• OME-Zarr file loading works correctly
• All existing tests pass
• No xtensor/xsimd in final build dependencies