Pleiades : improve OpticalCalibration (update gain/biases values, and correct some mistakes)

OpticalCalibration performs Pleaides calibration (from DN to TOA and TOC reflectance), but there are some points to fix (see also these messages):

1. user should be able to choose between three modes when providing gain/biases values : read gain/biases from the product (ie : from the DIMAP file, if available), take hard-coded tabulated values, read them from a user file (txt file, with two lines for gains & bias)
2. when reading gain/bias from a file, the application should not display hard-coded values. In current versions, it displays these hard-coded values even when user provides a specific file, but, I guess the computation is correct, because output images are (slightly) different when we provide (or not) the gain/bias file.
3. the sun and viewing angles should be taken from the center of the image (see user forum)
4. when providing a DIMAP as input of OpticalCalibration, the application should have the same behaviour as when we provide the corresponding .tif or .jp2 file. In our tests, when we provide a DIMAP, OTB does not recognize the sensor and needs a gain/bias file, whereas when we set the image.jp2 as input, it recognizes the sensor and use hard coded gain/bias

The Gain_PHR_ICC_20201001.xlsx file gives gain values that should be used for PHR1A and PHR1B calibration. These values have been provided by the CNES / Pleiades calibration team.

Edit (2021-01-05) : as reported by Etienne here , the documentation should be improved to explain the expected order of the bands.