add check for solver installation when initializing solver class instances

[daniel-rdt]

This PR adds a check to the initialization of a solver instance to check whether the solver package is installed or not. An ImportError is raised if the necessary package is not installed.
fyi @FabianHofmann @lkstrp

[lkstrp]

Can we not just use available_solvers instead of checking again?

linopy/linopy/solvers.py

Lines 50 to 115 in acac414

	available_solvers = []
	which = "where" if os.name == "nt" else "which"
	# the first available solver will be the default solver
	with contextlib.suppress(ModuleNotFoundError):
	import gurobipy
	available_solvers.append("gurobi")
	with contextlib.suppress(ModuleNotFoundError):
	_new_highspy_mps_layout = None
	import highspy
	available_solvers.append("highs")
	from importlib.metadata import version
	if version("highspy") < "1.7.1":
	_new_highspy_mps_layout = False
	else:
	_new_highspy_mps_layout = True
	if sub.run([which, "glpsol"], stdout=sub.DEVNULL, stderr=sub.STDOUT).returncode == 0:
	available_solvers.append("glpk")
	if sub.run([which, "cbc"], stdout=sub.DEVNULL, stderr=sub.STDOUT).returncode == 0:
	available_solvers.append("cbc")
	with contextlib.suppress(ModuleNotFoundError):
	import pyscipopt as scip
	available_solvers.append("scip")
	with contextlib.suppress(ModuleNotFoundError):
	import cplex
	available_solvers.append("cplex")
	with contextlib.suppress(ModuleNotFoundError):
	import xpress
	available_solvers.append("xpress")
	with contextlib.suppress(ModuleNotFoundError):
	import mosek
	with contextlib.suppress(mosek.Error):
	with mosek.Env() as m:
	t = m.Task()
	t.optimize()
	m.checkinall()
	available_solvers.append("mosek")
	with contextlib.suppress(ModuleNotFoundError):
	import mindoptpy
	available_solvers.append("mindopt")
	with contextlib.suppress(ModuleNotFoundError):
	import coptpy
	with contextlib.suppress(coptpy.CoptError):
	coptpy.Envr()
	available_solvers.append("copt")

[daniel-rdt]

Can we not just use available_solvers instead of checking again?

Hm, good point. Then we could just define the function in the abstract class to check if solver_name is in available_solvers and raise the error if not?

[lkstrp]

Yes, we wouldn't even need a extra method, just checking

if self.solver_name.value not in available_solvers:
    raise ImportError('..')

in the abc init would be enough.

And for each solver we could use the created enum:

self.solver_name = SolverName.xx

[daniel-rdt]

We could even not specify the name but just one abstract function to get the name from the class __name__ via the SolverName enum:

def get_name_str(self):
    return SolverName[self.__class__.__name__].value

I made the changes