Fix sparse storage of multidimensional tally results

[paulromano]

Description

This fixes conversion of tally result arrays to SciPy LIL sparse arrays. SciPy sparse arrays are two-dimensional, but tally data is logically shaped by filter, nuclide, and score bins. The previous conversion attempted to construct a sparse array using the tally's three-dimensional shape and fails with current SciPy versions with the following exception:

ValueError: LIL arrays don't support 1D input. Use 2D

This PR now now stores tally data internally as a two-dimensional row sparse array. A single row avoids the per-row Python-list overhead that LIL storage would incur for a column with one row per tally bin. Existing tally accessors continue to return dense arrays reshaped to the logical tally dimensions, so the public data representation is unchanged.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[nuclearkevin]

LGTM!