Update Laplace class and add unit tests (#645)

tests/test_problem_zoo/test_advection.py

@@ -5,7 +5,7 @@ from pina.problem import SpatialProblem, TimeDependentProblem
 
 @pytest.mark.parametrize("c", [1.5, 3])
 def test_constructor(c):
-    print(f"Testing with c = {c} (type: {type(c)})")
+
     problem = AdvectionProblem(c=c)
     problem.discretise_domain(n=10, mode="random", domains="all")
     assert problem.are_all_domains_discretised
@@ -14,5 +14,6 @@ def test_constructor(c):
     assert hasattr(problem, "conditions")
     assert isinstance(problem.conditions, dict)
 
+    # Should fail if c is not a float or int
     with pytest.raises(ValueError):
-        AdvectionProblem(c="a")
+        AdvectionProblem(c="invalid")