Image Processing docs structure based on "Principles of Digital Image Processing" book

### Overview
This is a task to organize **documentation** of our the **Image Processing** features in GIL following on structure of the topics as proposed in the 3-volume series of the **Principles of Digital Image Processing** book ([1](https://www.springer.com/gp/book/9781848001909), [2](https://www.springer.com/gp/book/9781848001947), [3](https://www.springer.com/gp/book/9781848829183)).

Simply, just following the table of contents of those books would help to come up with structure that is clean, maintainable, _future-proof_ and friendly to readers, those familiar with image processing concepts and GIL as well as newbies.

### Task

Below is proposal of the the "Image Processing table of contents.
Please, _feel free to comment and discuss_ it in the comments below or following the [thread on the boost-gil mailing list](https://lists.boost.org/boost-gil/2019/10/0306.php).

### ToC: Image Processing

- Histograms and Statistics
- Grayscale images
- Color images
- Basic Point Operations - _algebraic and logical operations to modify pixel values without changing size, geometry or structure of image_
- Intensity (contrast , brightness, inverting, simple global thresholding)
- Histogram Equalization
- Gamma Correction
- Color Space Transformations
- Filters (grayscale and color images)
- Linear
- Non-linear
- Morphological
- Classification and Segmentation - _methods for labeling, binarizing and finding_
- Thresholding
- Simple Global Thresholding
- Automatic Thresholding
- Global Adaptive Thresholding
- Local Adaptive Thresholding
- Labeling Regions
- Detectors - _methods for object detection_
- Edges
- Curves
- Corners
- Contours
- Geometric Operations - _operations altering image geometry_
- Image Transforms - _operators altering geometry and nature of the image information as well as geometry of the image itself_
- Distance Transform
- Fourier Transform (DFT, FFT)
- Cosine Transform (DCT)
- Haar Transform
- Hough Transform

### Changes History

Changes of the proposal above following comments and discussions from you!

1. Initial proposal
...