Adding new problems to problem.zoo (#484)

* adding problems
* add tests
* update doc + formatting

---------

Co-authored-by: Dario Coscia <dariocos99@gmail.com>

tests/test_problem_zoo/test_diffusion_reaction.py

@@ -0,0 +1,12 @@
+from pina.problem.zoo import DiffusionReactionProblem
+from pina.problem import TimeDependentProblem, SpatialProblem
+
+
+def test_constructor():
+    problem = DiffusionReactionProblem()
+    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)