Changed Array2D from_string constructor to be classmethod

[HarrisonTCodes]

Changed Array2D from_string constructor / factory method, which was previously a staticmethod, to be a classmethod.

This is inline with common semantics of constructor / factory methods being class methods. It also fixes previously broken usage of this method with inheritance and child classes.

Previously, this code:

class Test(Array2D):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
    def test(self):
        print("test")

t = Test.from_string("0,1,0\n0,0,1\n0,0,0")
t.test()

Would error, as the from_string static method always returns an instance of the parent class, and t would be of type Array2D instead of the child Test class, hence the test method would not exist on it. With the use of a class method instead, t = Test.from_string(...) returns an instance of the child class (Test) correctly.