feat: denote constant parameters when printing fit results

[alexander-held]

Add a (constant) flag when printing parameters that were held fixed in fits, which subsequently have zero uncertainty. This allows for more easily distinguishing them from the rest.

* denote constant parameters as such when printing fit results

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (063dcd0) 100.00% compared to head (7a883bb) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #407 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 22 22 Lines 2068 2069 +1 Branches 334 334 ========================================= + Hits 2068 2069 +1 ``` | [Impacted Files](https://app.codecov.io/gh/scikit-hep/cabinetry/pull/407?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/fit/\_\_init\_\_.py](https://app.codecov.io/gh/scikit-hep/cabinetry/pull/407?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep#diff-c3JjL2NhYmluZXRyeS9maXQvX19pbml0X18ucHk=) | `100.00% <100.00%> (ø)` | |

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