Handle 64-bit images

[koalive]

Currently, 64-bit TIFF images cannot be opened by FileIO. The current workaround is to convert each image to 32-bit TIFF files externally (e.g. with tifffile in Python).

[koalive]

Recent versions of the dependencies are able to handle 64-bit images, e.g.:

ImageMagick v0.7.9
Images v0.24.1

while older ones return arrays of NaN, such as

ImageMagick v0.7.5
Images v0.17.3

Some tests regarding the loading of 8- to 32-bit TIFF were added in 8eb946e. In release 1.2, they should be expanded to test 64-bit images as well, and the dependencies should be adapted.