smurf: Add utility to merge FITS headers

The new application FITSMERGE merges the FITS headers in its input
files and writes the merged header into another file.  This is
to make the SMURF routine smf_fits_outhdr available to PICARD.
smurf: Keep OBSGEO headers from the end of group

This commit adds the OBSGEO-X, OBSGEO-Y and OBSGEO-Z headers to the
list of headers which the smf_fits_outhdr function retains from the
end of the group of input headers if they differ.  This is to prevent
these headers disappearing when merging headers from different
observations between which the position of the telescope changed.