Pixmap image as input for collimator
Let the users specify a black and white image that describes the hole of the collimator. An example of a black and white image is attached.
As input file format Portable Bitmap is suggested since a parser is relatively simple to implement and we'll avoid to rely on yet another library.
The simplest way to implement this is to extend the FlexibleCollimator and use one rectangle for each black pixel. However this isn't ideal because the number of rectangles can be big. Since we use a quadtree the performance may not be so bad. We'll have to test it.
Substantially more effort is needed if we try to minimize the number of rectangles by clustering pixels.
Submitted by @ext-roussel_r
