Read in a pixel mask

[briantoby]

GSAS-II has a lot of ways to remove spurious sections of an image prior to image integration (in the Masks section) and had the ability to scan an image for spurious intensities to create a "pixel mask" of individual pixels that should not be used, but the request was made by APS 1-ID users to be able to create this pixel mask externally (see #227). This has been added. The mask file is imported in as a standard GSAS-II image, where every non-zero value in the mask is considered as a pixel to be ignored. In the Masks data tree item, there is a menu command to designate an image as a pixel mask. There is also a scripting command to do the same.