Fix/128

[hombit]

Fix #128: find min and max values ignoring NaN

Check this GUI features before merge:

• [x] Positive and negative BITPIX images
• [x] Zoom, rotate and flip
• [x] Level control
• [x] Color maps
• [x] Open new image in the same window and in new window

[matwey]

Could you please squash 2nd and 4th commits together?