Refactor problems

examples/standalone/problem.py

@@ -1,6 +1,6 @@
 import numpy as np
 
-from pybop._problem import BaseProblem
+from pybop import BaseProblem
 
 
 class StandaloneProblem(BaseProblem):

pybop/__init__.py

@@ -31,7 +31,9 @@
 # Cost class
 # Problem class
 #
-from ._problem import BaseProblem, FittingProblem, DesignProblem
+from .problems.base_problem import BaseProblem
+from .problems.fitting_problem import FittingProblem
+from .problems.design_problem import DesignProblem
 
 #
 # Cost function class