Inconsistent handling of focus point coordinates in mapping files

[capricorn8]

Handling of focus point coordinates in mapping .txt files (for Nikon and Pentax) is inconsistent and confusing which can lead to wrong placement of focus points on the image.

For Pentax, coordinates represent the CENTER of the AF point. This is documented in every mapping file for a specific camera/sensor type and the Pentax-specific processing interprets the (x,y) values correspondingly.

For Nikon, coordinates should represent the TOP LEFT corner of the AF point. For D700, the mapping files uses CENTER coordinates which results in shifted visualization compared to NX Studio.

Tasks:

1. Fix the D700 issue
2. Document the Nikon requirement on mapping files in a prominent place,
3. Optionally, unify handling of focus point coordinates across brands

[capricorn8]

All Nikon focus point name -> coordinates mapping files will be changed to CENTER representation for V2.2.

This representation eases the creation of high precision mapping files by means of an Excel sheet:

1. Use exiftool to write the complete PDAF point layout of the sensor (i.e. all relevant focus point names) to an image file
2. Use NX studio to display the "grid" of focus points
3. Take measurements (counting pixels) of basic parameters (focus box sizes and spacing between boxes)
4. Enter the parameters into an Excel template prepared for the related sensor and generate the .txt file
5. Visual check of the grid displayed by the plugin against NX Studio display, and fine-tuning if necessary