Bug fix to Stack repr which also broke resp repr

[codecov-commenter]

Codecov Report

Base: 95.25% // Head: 100.00% // Increases project coverage by +4.74% :tada:

Coverage data is based on head (feee237) compared to base (304983a). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #45 +/- ## =========================================== + Coverage 95.25% 100.00% +4.74% =========================================== Files 7 7 Lines 253 253 =========================================== + Hits 241 253 +12 + Misses 12 0 -12 ``` | [Impacted Files](https://codecov.io/gh/ehsteve/roentgen/pull/45?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Steven+Christe) | Coverage Δ | | |---|---|---| | [roentgen/absorption/material.py](https://codecov.io/gh/ehsteve/roentgen/pull/45?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Steven+Christe#diff-cm9lbnRnZW4vYWJzb3JwdGlvbi9tYXRlcmlhbC5weQ==) | `100.00% <100.00%> (+8.51%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Steven+Christe). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Steven+Christe)

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[ehsteve]

Thanks. I'll fix. I never understand the difference between repr and str so if you have any suggestions let me know.

[ehsteve]

@samaloney made updates based on your comments!