Added 'rotate_vars' to props at load-time.

This commit standardizes the way rotations are handled. The
'rotate_vars' property is now a set, and we always use it to as the list
of variables to rotate. Also, this includes a fix so that the
orientation matrix always represents the instrument orientation in the
current reference frame (as long as the adv.rotate functions are used).
BugFix GN2002.

This tool now handles large NaN gaps more cleanly. It splits the record
up where there are large gaps (>Npt/10).