Consolidate getitem functionality

[FabianHofmann]

From release notes:

• The classes Variable, LinearExpression and Constraint now have a new getitem method that allows selecting a subset of the object in the same way as xarray objects, i.e. by integer labels or boolean index. Example usage: x[[1, 2]] or x[x.indexes["some_index"] > 5].

• The class Constraint now has a new method .loc to select a subset of the constraint by labels.

• Selecting a single variable with the getitem ([]) method now raises a FutureWarning that the return type will change to Variable instead of a ScalarVariable in the future. To get a ScalarVariable in the future, use the at[] method.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.11765% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 88.97%. Comparing base (f5f6144) to head (78511c5).

Files	Patch %	Lines
linopy/variables.py	92.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #285 +/- ## ========================================== + Coverage 88.94% 88.97% +0.03% ========================================== Files 15 15 Lines 3689 3711 +22 Branches 870 876 +6 ========================================== + Hits 3281 3302 +21 Misses 283 283 - Partials 125 126 +1 ```

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