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

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

pina/equation/equation_factory.py

@@ -0,0 +1,37 @@
+""" Module """
+from .equation import Equation
+from ..operators import grad, div, nabla
+
+
+class FixedValue(Equation):
+    
+    def __init__(self, value, components=None):
+        def equation(input_, output_):
+            if components is None:
+                return output_ - value
+            return output_.extract(components) - value 
+        super().__init__(equation)
+
+
+class FixedGradient(Equation):
+
+    def __init__(self, value, components=None, d=None):
+        def equation(input_, output_):
+            return grad(output_, input_, components=components, d=d) - value
+        super().__init__(equation)
+
+
+class FixedFlux(Equation):
+
+    def __init__(self, value, components=None, d=None):
+        def equation(input_, output_):
+            return div(output_, input_, components=components, d=d) - value
+        super().__init__(equation)
+
+
+class Laplace(Equation):
+
+    def __init__(self, components=None, d=None):
+        def equation(input_, output_):
+            return nabla(output_, input_, components=components, d=d)
+        super().__init__(equation)