Add function for querying attributes of solver model

[Irieo]

The new function get_solver_attribute() introduces a mechanism for querying properties of a solver model through the attribute interface. This extends the functionality of Linopy interface by allowing users to retrieve properties from mathematical model, variable attributes and attributes associated with solution quality -> and map this information to dimensions of a linopy problem.

For now the functionality works with Gurobi interface. The list of attributes can be found here. NB some attributes are available at all times (e.g., the number of variables in the model), some are only populated at certain times (e.g., those related to the problems of a specific type).

The functionality can be extended to interfaces of other solvers if Linopy & PyPSA communities finds this useful.

The original idea driving this PR is to create convenient mechanism to retrieve & map to dimensions of an original linopy problem sensitivity information related to variables (SAObjLow, SAObjUp, RC) that is promising to be useful for both debugging and analysis for PyPSA applications.

Toy example:

import pypsa, pandas as pd
n = pypsa.Network()
n.add("Bus", "DE", v_nom=380)
n.add("Generator", "gas", bus="DE", marginal_cost=70, p_nom=50)
n.add("Generator", "coal", bus="DE", marginal_cost=40, p_nom=70)
n.snapshots = pd.date_range("2019-01-01", periods=5, freq="H")
load_series = pd.Series([30, 60, 80, 100, 70], index=n.snapshots)
n.add("Load", "Germany", bus="DE", p_set=load_series)
n.add("StorageUnit", "Battery", bus="DE", p_nom=20, max_hours=5, efficiency_store=1, efficiency_dispatch=0.9, state_of_charge_initial=0)
n.optimize(solver_name="gurobi")
n.model.variables.get_solver_attribute("SAObjUp")

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 88.94%. Comparing base (3bd24bc) to head (6c083d6).

Files	Patch %	Lines
linopy/variables.py	88.23%	2 Missing ⚠️

Additional details and impacted files

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- Coverage   88.94%   88.94%   -0.01%     
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  Files          15       15              
  Lines        3645     3663      +18     
  Branches      856      861       +5     
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+ Hits         3242     3258      +16     
- Misses        279      281       +2     
  Partials      124      124              

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