IO: multiindeces and general fixes

[FabianHofmann]

• The IO with NetCDF files was made more secure and fixed for some cases. In particular, variables and constraints with a dash in the name are now supported (as used by PyPSA). The object sense and value are now properly stored and retrieved from the netcdf file.
• The IO with NetCDF file now supports multiindexed coordinates.
• The representation of single indexed expressions and constraints with non-empty dimensions/coordinates was fixed, e.g. x.loc[["a"]] > 0 where x has only one dimension. Therefore the representation now shows the coordinates.

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (b437f93) 89.17% compared to head (2ac42c5) 89.23%. Report is 4 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #202 +/- ## ========================================== + Coverage 89.17% 89.23% +0.05% ========================================== Files 15 15 Lines 3298 3343 +45 Branches 757 768 +11 ========================================== + Hits 2941 2983 +42 Misses 248 248 - Partials 109 112 +3 ``` | [Files](https://app.codecov.io/gh/PyPSA/linopy/pull/202?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/202?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L2NvbW1vbi5weQ==) | `92.50% <ø> (+0.83%)` | :arrow_up: | | [linopy/constraints.py](https://app.codecov.io/gh/PyPSA/linopy/pull/202?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L2NvbnN0cmFpbnRzLnB5) | `90.66% <100.00%> (-0.51%)` | :arrow_down: | | [linopy/expressions.py](https://app.codecov.io/gh/PyPSA/linopy/pull/202?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L2V4cHJlc3Npb25zLnB5) | `89.44% <100.00%> (-0.31%)` | :arrow_down: | | [linopy/testing.py](https://app.codecov.io/gh/PyPSA/linopy/pull/202?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L3Rlc3RpbmcucHk=) | `100.00% <100.00%> (ø)` | | | [linopy/io.py](https://app.codecov.io/gh/PyPSA/linopy/pull/202?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L2lvLnB5) | `88.32% <95.12%> (+0.41%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/PyPSA/linopy/pull/202/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA)

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