feat: add error message if highspy not installed (#491)

Author: FabianHofmann

linopy/solvers.py

@@ -458,6 +458,10 @@ def solve_problem_from_file(
         status.legacy_status = first_line
 
         # Use HiGHS to parse the problem file and find the set of variable names, needed to parse solution
+        if "highs" not in available_solvers:
+            raise ModuleNotFoundError(
+                f"highspy is not installed. Please install it to use {self.solver_name.name} solver."
+            )
         h = highspy.Highs()
         h.silent()
         h.readModel(path_to_string(problem_fn))