Update Laplace class and add unit tests (#645)

tests/test_problem_zoo/test_diffusion_reaction.py

@@ -1,12 +1,19 @@
+import pytest
 from pina.problem.zoo import DiffusionReactionProblem
 from pina.problem import TimeDependentProblem, SpatialProblem
 
 
-def test_constructor():
-    problem = DiffusionReactionProblem()
+@pytest.mark.parametrize("alpha", [0.1, 1])
+def test_constructor(alpha):
+
+    problem = DiffusionReactionProblem(alpha=alpha)
     problem.discretise_domain(n=10, mode="random", domains="all")
     assert problem.are_all_domains_discretised
     assert isinstance(problem, TimeDependentProblem)
     assert isinstance(problem, SpatialProblem)
     assert hasattr(problem, "conditions")
     assert isinstance(problem.conditions, dict)
+
+    # Should fail if alpha is not a float or int
+    with pytest.raises(ValueError):
+        problem = DiffusionReactionProblem(alpha="invalid")