Modify pina.__init__.py and rm useless .py files

* rm meta.py, plotter.py, writer.py
* modify __init__ file
* modify tests due to __init__ import

tests/test_optimizer.py

@@ -1,6 +1,6 @@
 import torch
 import pytest
-from pina import TorchOptimizer
+from pina.optim import TorchOptimizer
 
 opt_list = [
     torch.optim.Adam, torch.optim.AdamW, torch.optim.SGD, torch.optim.RMSprop