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

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

pina/equation/equation.py

@@ -0,0 +1,10 @@
+""" Module """
+from .equation_interface import EquationInterface
+
+class Equation(EquationInterface):
+
+    def __init__(self, equation):
+        self.__equation = equation
+
+    def residual(self, input_, output_):
+        return self.__equation(input_, output_)