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_helmholtz.py

@@ -0,0 +1,16 @@
+import pytest
+from pina.problem.zoo import HelmholtzProblem
+from pina.problem import SpatialProblem
+
+
+@pytest.mark.parametrize("alpha", [1.5, 3])
+def test_constructor(alpha):
+    problem = HelmholtzProblem(alpha=alpha)
+    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)
+
+    with pytest.raises(ValueError):
+        HelmholtzProblem(alpha="a")