equation class, fix minor bugs, diff domain (#89)

* equation class
* difference domain
* dummy dataloader
* writer class
* refactoring and minor fix

pina/equation/system_equation.py

@@ -0,0 +1,24 @@
+""" Module """
+import torch
+from .equation import Equation
+
+class SystemEquation(Equation):
+
+    def __init__(self, list_equation):
+        if not isinstance(list_equation, list):
+            raise TypeError('list_equation must be a list of functions')
+
+        self.equations = []
+        for i, equation in enumerate(list_equation):
+            if not callable(equation):
+                raise TypeError('list_equation must be a list of functions')
+            
+            self.equations.append(Equation(equation))
+
+    def residual(self, input_, output_):
+        return torch.mean(
+            torch.stack([
+                equation.residual(input_, output_)
+                for equation in self.equations
+            ]),
+            dim=0)