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

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

pina/writer.py

@@ -0,0 +1,54 @@
+""" Module for plotting. """
+import matplotlib.pyplot as plt
+import numpy as np
+import torch
+
+from pina import LabelTensor
+
+
+class Writer:
+    """
+    Implementation of a writer class, for textual output.
+    """
+
+    def __init__(self,
+                 frequency_print=10,
+                 header='any') -> None:
+
+        """
+        The constructor of the class.
+
+        :param int frequency_print: the frequency in epochs of printing.
+        :param ['any', 'begin', 'none'] header: the header of the output.
+        """
+
+        self._frequency_print = frequency_print
+        self._header = header
+
+
+    def header(self, trainer):
+        """
+        The method for printing the header.
+        """
+        header = []
+        for condition_name in trainer.problem.conditions:
+            header.append(f'{condition_name}')
+
+        return header
+
+    def write_loss(self, trainer):
+        """
+        The method for writing the output.
+        """
+        pass
+
+
+    def write_loss_in_loop(self, trainer, loss):
+        """
+        The method for writing the output within the training loop.
+
+        :param pina.trainer.Trainer trainer: the trainer object.
+        """
+
+        if trainer.trained_epoch % self._frequency_print == 0:
+            print(f'Epoch {trainer.trained_epoch:05d}: {loss.item():.5e}')