bioinspired.Retina: improve logSampling - custom center & diameter of fovea

[breznak]

System information (unrelated)

Ubuntu 18.04 LTS linux, python 3.7,
opencv-contrib installed from pypi: opencv-contrib-python (4.1.1.26)

Detailed description

Feature request/HOWTO:

• I'm using cv2.bioinspired.Retina model with useRetinaLogSampling=True
• I'd like to be able to process images of arbitrary size (rectangural)
• I'd like to specify position & fovea diameter

Steps to reproduce

use cv2.bioinspired.Retina with useRetinaLogSampling=True,

• the center of fovea is in the center of the image
  • F1: I'd like to make this customizable
• the radius of the fovea is maximum to the scene/image
  • F2: make this customizable

For reference, you can see my source using Retina:
htm-community/htm.core#691