constant: ensure float value & improve printing of constants

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (d43b533) 89.15% compared to head (b6fc7eb) 89.17%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #194 +/- ## ========================================== + Coverage 89.15% 89.17% +0.01% ========================================== Files 15 15 Lines 3291 3297 +6 Branches 753 756 +3 ========================================== + Hits 2934 2940 +6 Misses 248 248 Partials 109 109 ``` | [Files](https://app.codecov.io/gh/PyPSA/linopy/pull/194?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA) | Coverage Δ | | |---|---|---| | [linopy/common.py](https://app.codecov.io/gh/PyPSA/linopy/pull/194?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L2NvbW1vbi5weQ==) | `91.66% <100.00%> (+0.41%)` | :arrow_up: | | [linopy/expressions.py](https://app.codecov.io/gh/PyPSA/linopy/pull/194?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L2V4cHJlc3Npb25zLnB5) | `89.74% <100.00%> (+0.03%)` | :arrow_up: | | [linopy/objective.py](https://app.codecov.io/gh/PyPSA/linopy/pull/194?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L29iamVjdGl2ZS5weQ==) | `93.68% <100.00%> (ø)` | | | [linopy/solvers.py](https://app.codecov.io/gh/PyPSA/linopy/pull/194?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L3NvbHZlcnMucHk=) | `89.11% <83.33%> (-0.12%)` | :arrow_down: |

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