add 'dirname' gnulib module
update gnulib
support to write MRC stacks

gfunc3_to_mrc() takes now an additional FILE ** argument to keep the
stream open. With the new gfunc3_write_to_stack(), it is possible to
write image by image to disk.
add et_operators.h to ETreco.h
fix output file name determination
protect from multiple inclusion
add detector shape parameter to struct
WIP: implement forward operator
separate AI and ET functions
change stack handling in MRC
rewrite for single-precision NFFT
minor changes
WIP: at-once version of forward operator