Small fix to SIFT debugging image output
tckmap: Strip template header comments

Comments from an image used as a template for producing a TWI may not be relevant to the TWI itself, and only lead to a bloated header (especially if one TWI is used as the template for another).
This change strips away any comments read from the header of the template image.
tcksift: Added selection mask output

Added an output option to SIFT that writes a text file indicating which streamlines were retained and removed from the reconstruction.