Runner: run_solver.py update with knitro (#410)

The goal of this pull request is add support for the `knitro` solver in
the `run_solver.py` script.

The interface between `linopy` and the `knitro` Python API is introduced
in this [pull request.](PyPSA/linopy#532)

## Changes

### `run_solver.py`

I have added a `knitro` specification for the following functions:
- `def get_solver`
- `def def is_mip_problem`
- `def get_reported_runtime`
- `def get_duality_gap`

## Test
I tested the integration between knitro and linopy and the changes to
run_solver.py on the benchmark
[pypsa-eur-elec-trex-3-12h.lp](https://storage.googleapis.com/solver-benchmarks/pypsa-eur-elec-trex-3-12h.lp).

I get:
`{"runtime": 4.476548100999992, "reported_runtime": 3.8160810470581055,
"status": "ok", "condition": "optimal", "objective": 7398198432.386027,
"duality_gap": null, "max_integrality_violation": null}`

The value of the objective function is compatible to those found
[here](https://openenergybenchmark.org/dashboard/benchmark-set/pypsa-eur-elec-trex)

---------

Co-authored-by: Daniele Lerede <daniele.lerede@openenergytransition.org>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

runner/run_solver.py

@@ -76,6 +76,9 @@ def get_solver(solver_name):
             "randomseed": 0,
             "mip.tolerances.mipgap": mip_gap,
         },
+        "knitro": {  # TODO check seed option for knitro
+            "KN_PARAM_MS_SEED": 1066,
+        },
     }
 
     return solver_class(**seed_options.get(solver_name, {}))
@@ -103,6 +106,9 @@ def is_mip_problem(solver_model, solver_name):
         # so MIP problem detection is not possible.
         # TODO preprocess benchmarks and add this info to metadata
         return False
+    elif solver_name == "knitro":
+        # Knitro is not designed for MILP problems
+        return False
     else:
         raise NotImplementedError(f"The solver '{solver_name}' is not supported.")
 
@@ -135,6 +141,9 @@ def get_duality_gap(solver_model, solver_name: str):
         return None
     elif solver_name == "cplex":
         return solver_model.solution.MIP.get_mip_relative_gap()
+    elif solver_name == "knitro":
+        # Knitro duality gap retrieval not implemented yet
+        return None
     else:
         raise NotImplementedError(f"The solver '{solver_name}' is not supported.")
 
@@ -180,6 +189,8 @@ def get_reported_runtime(solver_name, solver_model) -> float | None:
                 return solver_model.Runtime
             case "cplex":
                 return None
+            case "knitro":
+                return solver_model.reported_runtime
             case _:
                 print(f"WARNING: cannot obtain reported runtime for {solver_name}")
                 return None