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_poisson_2d_square.py

@@ -1,5 +1,11 @@
 from pina.problem.zoo import Poisson2DSquareProblem
+from pina.problem import SpatialProblem
 
 
 def test_constructor():
-    Poisson2DSquareProblem()
+    problem = Poisson2DSquareProblem()
+    problem.discretise_domain(n=10, mode="random", domains="all")
+    assert problem.are_all_domains_discretised
+    assert isinstance(problem, SpatialProblem)
+    assert hasattr(problem, "conditions")
+    assert isinstance(problem.conditions, dict)