Enhancing Equations

- add init file
- add docs
- fixing bug System of equation, replace torch.stack with torch.hstack
- add tests

pina/equation/equation.py

@@ -4,7 +4,29 @@ from .equation_interface import EquationInterface
 class Equation(EquationInterface):
 
     def __init__(self, equation):
+        """
+        Equation class for specifing any equation in PINA.
+        Each ``equation`` passed to a ``Condition`` object
+        must be an ``Equation`` or ``SystemEquation``. 
+
+        :param equation: A ``torch`` callable equation to
+            evaluate the residual.
+        :type equation: callable
+        """
+        if not callable(equation):
+            raise ValueError('equation must be a callable function.'
+                             'Expected a callable function, got '
+                             f'{equation}')
         self.__equation = equation
 
     def residual(self, input_, output_):
+        """
+        Residual computation of the equation.
+
+        :param LabelTensor input_: Input points to evaluate the equation.
+        :param LabelTensor output_: Output vectors given my a model (e.g,
+            a ``FeedForward`` model).
+        :return: The residual evaluation of the specified equation.
+        :rtype: LabelTensor
+        """
         return self.__equation(input_, output_)