make all seq commands consistent

Review all the commands applicable to sequences:

command	argument	comment	Done
getref	seqname	change to sequencename	[x]
light_curve	sequencename		[x]
merge	sequence1	no change required	[x]
preprocess	sequencename		[x]
register	sequence	change to sequencename	[x]
select		add sequencename argument + make scriptable	[x]
seqapplyreg	sequencename		[x]
seqclean	sequencename		[x]
seqcosme	sequencename		[x]
seqcosme_cfa	sequencename		[x]
seqcrop	sequencename		[x]
seqextract_Green	sequencename		[x]
seqextract_Ha	sequencename		[x]
seqextract_HaOIII	sequencename		[x]
seqheader	sequencename		[x]
seqfind_cosme	sequencename		[x]
seqfind_cosme_cfa	sequencename		[x]
seqfindstar	sequencename		[x]
seqfixbanding	sequencename		[x]
seqmerge_cfa	sequencename		[x]
seqmtf	sequencename		[x]
seqpsf	[sequencename	~~add required sequencename argument ~~	[ ]
seqplatesolve	sequencename		[x]
seqrl	sequence	change to sequencename	[x]
seqsb	sequence	change to sequencename	[x]
seqsplit_cfa	sequencename		[x]
seqstarnet	sequence	change to sequencename	[x]
seqstat	sequencename		[x]
seqsubsky	sequencename		[x]
seqtilt	[sequencename]	add required sequencename argument	[x]
seqwiener	sequence		[x]
setmagseq	magnitude	add sequencename argument + rename to seqsetmag	[x]
unselect		add sequencename argument + make scriptable	[x]
unsetmagseq		add sequencename argument + rename to sequnsetmag	[x]

Make sure they all call load_sequence so that . is a valid argument (replaces loaded sequence with its name).
Make sure they all have the same consistent sequencename argument.
Make sure sequencename argument is never optional

Note: seqpsf has been left out of this round of review. sequencename is still optional. Will reconsider in the future.