Renaming

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

pina/problem/zoo/diffusion_reaction.py

@@ -5,7 +5,7 @@ from pina import Condition
 from pina.problem import SpatialProblem, TimeDependentProblem
 from pina.equation.equation import Equation
 from pina.domain import CartesianDomain
-from pina.operators import grad
+from pina.operator import grad
 
 def diffusion_reaction(input_, output_):
     """