fix: handling spaces in histogram names for template visualization

[alexander-held]

The presence of spaces in region / sample / systematic names previously caused the corresponding templates to not get picked up via visualize.templates, which is now fixed.

* fix visualize.templates not correctly handling template histograms with spaces in names

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (5ece244) 100.00% compared to head (7dddca0) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #423 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 22 22 Lines 2072 2072 Branches 338 338 ========================================= Hits 2072 2072 ``` | [Impacted Files](https://app.codecov.io/gh/scikit-hep/cabinetry/pull/423?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | Coverage Δ | | |---|---|---| | [src/cabinetry/visualize/\_\_init\_\_.py](https://app.codecov.io/gh/scikit-hep/cabinetry/pull/423?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep#diff-c3JjL2NhYmluZXRyeS92aXN1YWxpemUvX19pbml0X18ucHk=) | `100.00% <ø> (ø)` | |

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