Inverse problem implementation (#177)

* inverse problem implementation

* add tutorial7 for inverse Poisson problem

* fix doc in equation, equation_interface, system_equation

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Co-authored-by: Dario Coscia <dariocoscia@dhcp-015.eduroam.sissa.it>

pina/equation/equation_interface.py

@@ -11,3 +11,17 @@ class EquationInterface(metaclass=ABCMeta):
     the output variables, the condition(s), and the domain(s) where the
     conditions are applied.
     """
+
+    @abstractmethod
+    def residual(self, input_, output_, params_):
+        """
+        Residual computation of the equation.
+
+        :param LabelTensor input_: Input points to evaluate the equation.
+        :param LabelTensor output_: Output vectors given by my model (e.g., a ``FeedForward`` model).
+        :param dict params_: Dictionary of unknown parameters, eventually
+            related to an ``InverseProblem``.
+        :return: The residual evaluation of the specified equation.
+        :rtype: LabelTensor
+        """
+        pass