source files: xmri.[ch] ge.[ch]
(XMRI-U-INT <file-header-string> <field-description>)
This function returns the value of the given field in the given file header. The first field description entries must be of the form
(GE-U-INT 1 ... )
BUG: 32-bit slisp doesn't actually have unsigned ints, so large unsigneds will return as negatives. If this becomes a significant problem, we can provide a function which returns them as slisp flonums, which are 'double' precision.