Renaming

* solvers -> solver
* adaptive_functions -> adaptive_function
* callbacks -> callback
* operators -> operator
* pinns -> physics_informed_solver
* layers -> block

tests/test_equations/test_systemequation.py

@@ -1,5 +1,5 @@
 from pina.equation import SystemEquation
-from pina.operators import grad, laplacian
+from pina.operator import grad, laplacian
 from pina import LabelTensor
 import torch
 import pytest