RF: refactor parse_PARREC into smaller functions

Makes testing easier.  I think the code is easier to read.
RF: parrec2nii uses one_line function from parrec

Now parrec has one_line function, use that instead of version defined in
parrec2nii.
RF: refactor process of image lines

Simplify logic of processing lines -- I didn't understand it so I found
myself rewriting.
RF: remove unused calculated values

Remove values returned from _calc_extra that are not used elsewhere in
the code.