Expose `AmrCore` Again

[ax3l]

In order to let users implement their whole simulation in pyAMReX (e.g. this user question), we could expose AmrCore again.

Here is the logic to implement purely virtual and python-overwritable methods:

• https://pybind11.readthedocs.io/en/stable/advanced/classes.html#overriding-virtual-functions-in-python
• https://pybind11.readthedocs.io/en/stable/advanced/classes.html#virtual-and-inheritance