Bug fixes for sw_fitpowder

[RichardWaiteSTFC]

Thanks @mducle for spotting these - I've also fixed a bug in evaluating chi-sq if there are empty bins present.

[github-actions[bot]]

Test Results

    4 files  ±0    112 suites  ±0   4m 38s :stopwatch: -11s   713 tests +2    695 :white_check_mark: +2  18 :zzz: ±0  0 :x: ±0  2 024 runs  +8  1 988 :white_check_mark: +8  36 :zzz: ±0  0 :x: ±0 

Results for commit 6271d71e. ± Comparison against base commit 3705737b.

This pull request removes 2 and adds 4 tests. Note that renamed tests count towards both.

``` sw_tests.unit_tests.unittest_sw_fitpowder ‑ test_crop_energy_range sw_tests.unit_tests.unittest_sw_fitpowder ‑ test_exclude_energy_range ``` ``` sw_tests.unit_tests.unittest_sw_fitpowder ‑ test_add_1Dcuts_after_2D_data sw_tests.unit_tests.unittest_sw_fitpowder ‑ test_crop_energy_range_1d sw_tests.unit_tests.unittest_sw_fitpowder ‑ test_crop_energy_range_2d sw_tests.unit_tests.unittest_sw_fitpowder ‑ test_exclude_energy_range_1d ```