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

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

pina/geometry/location.py

@@ -0,0 +1,17 @@
+"""Module for Location class."""
+
+from abc import ABCMeta, abstractmethod
+
+
+class Location(metaclass=ABCMeta):
+    """
+    Abstract Location class.
+    Any geometry entity should inherit from this class.
+    """
+    @abstractmethod
+    def sample(self):
+        """
+        Abstract method for sampling a point from the location. To be
+        implemented in the child class.
+        """
+        pass