Add BigPixelDetector a detector with with a configurable set of big pixels

Adding an inherited detector class, BigPixelDetector, a detector with a configurable set of big pixels. The big pixels have twice the size of normal pixels. Big pixels are treated as two normal pixels and their coordinates are converted to the row, column, global position, and local positions accordingly.

The user has to specify the detector with big pixels by using big pixel coordinates (i.e. coordinates = "cartesian_big"). And the pixel # of those which are twice the size in x and y (i.e. big_pixel = [[0,51,52],[50,51]]).

Have not yet finished adding the information into the user manual, currently considering adding it in chapter 3.6 and also in chapter 4.3.