Version Checks (indicate both or one)

• I have confirmed this bug exists on the lastest release of Linopy.

• I have confirmed this bug exists on the current master branch of Linopy.

Issue Description

If you try to use CBC solver without highspy installed you get a vague "NameError"

If you try to pass a variable to add_objective, it gets caught by the type checker

Reproducible Example

# Example 1
import linopy
model = linopy.Model()
x = model.add_variables(name="x", lower=0.0)
model.add_constraints(x >= 0.0)
model.add_objective(x*1.0, sense="min")
model.solve(solver_name="cbc")  # NameError: name 'highspy' is not defined

# Example 2
import linopy
model = linopy.Model()
x = model.add_variables(name="x", lower=0.0)
model.add_constraints(x >= 0.0)
model.add_objective(x, sense="min")  # ValueError: Invalid type of `expr`

Expected Behavior

I expect to get a useful message to say that highspy should be installed to use cbc
I expect to be able to pass a Variable to the add_objective message

Installed Versions