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

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

pina/equation/equation_interface.py

@@ -0,0 +1,13 @@
+""" Module for EquationInterface class """
+from abc import ABCMeta, abstractmethod
+
+
+class EquationInterface(metaclass=ABCMeta):
+    """
+    The abstract `AbstractProblem` class. All the class defining a PINA Problem
+    should be inheritied from this class.
+
+    In the definition of a PINA problem, the fundamental elements are:
+    the output variables, the condition(s), and the domain(s) where the
+    conditions are applied.
+    """