[Self consistency] Replace DataFrame-based tally conversion with n-D OFB tallies

[SteSeg]

[Self consistency] Replace DataFrame-based tally conversion with nD OFB tallies

Summary

This PR removes the intermediate pandas.DataFrame step when converting OpenMC tallies from a statepoint file to benchmark_results.h5.

Previously, tallies were converted as:

OpenMC tally -> pandas.DataFrame -> xarray.DataArray -> HDF5

Because DataFrames are inherently 2D, this imposed shape limitations and made it harder to preserve full tally structure.

With this PR, tallies are handled as true multi-dimensional arrays, consistent with OpenMC and with the OFB specification model.

What changes

• Introduced BaseTally and Tally objects for OFB tally handling.
• Added src/openmc_fusion_benchmarks/backends/ to support transport-code-specific conversions.
• Implemented OpenMC backend conversion in src/openmc_fusion_benchmarks/backends/openmc/.
• Updated result handling so dimensions are preserved according to the benchmark specification:
  • score
  • nuclide
  • filters
  • and any additional dimensions required by the spec.

Why this matters

• Preserves physical/data structure of tallies end-to-end.
• Removes artificial 2D constraints from the conversion pipeline.
• Establishes a backend abstraction for future transport codes.
• Aligns benchmark_results.h5 tally representation with OFB specification semantics.

Usage example

import openmc_fusion_benchmarks as ofb

# Load OFB benchmark results
br = ofb.BenchmarkResults("benchmark_results.h5")

# List available tallies
print(br.tallies)

# Retrieve one tally by name
my_tally = br.get_tally(name="mytally")

# Access mean values (xarray-backed)
mean = my_tally.mean

# Inspect shape
print(mean.shape)

Notes for reviewers

This PR is part of a stacked series.
Related PRs in this series use the [Self consistency] prefix.