Renaming

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

pina/callback/__init__.py

@@ -0,0 +1,10 @@
+__all__ = [
+    "SwitchOptimizer",
+    "R3Refinement",
+    "MetricTracker",
+    "PINAProgressBar",
+]
+
+from .optimizer_callback import SwitchOptimizer
+from .adaptive_refinment_callback import R3Refinement
+from .processing_callback import MetricTracker, PINAProgressBar