Class to enable a custom image similarity metric defined in Python

## Description

Enable creation of a custom similarity metric in Python. See the discussion here:

https://discourse.itk.org/t/is-there-a-way-to-set-custom-metric-in-itk-registration-in-python/705/7

### Expected behavior

A convenient way to define an image similarity metric in Python that re-uses the ITK image registration frame, i.e. optimizers, transformations, multi-resolution capabilities, regularization.

### Actual behavior

The registration metric must be implemented in C++.

### Additional Information

Possible implementation:

An `itk.PythonImageRegistrationMetricv4` class would be similar to [itk.ImageToImageMetricv4](https://itk.org/Doxygen/html/classitk_1_1ImageToImageMetricv4.html), but it would have additional methods,

- `GetValuePythonCallback`
- `GetDerivativePythonCallback`
- `GetValueAndDerivativePythonCallback`

These methods would be passed Python functions that accept the `ImageToImageMetricv4` as an argument and would need to return required value and derivative values.

See also the class for defining a custom ITK filter in Python:

https://github.com/InsightSoftwareConsortium/ITK/blob/master/Wrapping/Generators/Python/PyUtils/itkPyImageFilter.h