Update pipeline doc and mindboggle-101 script
Add largest_segment option to spectra and zernike *_per_label functions.

Boolean to decide whether to only compute for largest segment with a given label.
Add select_largest() to select the largest segment (connected set of indices) in a mesh
Add select_largest() function to spectrum_of_largest() function (remove redundancy).
Confirm addition of largest_segment option to spectra and zernike *_per_label functions gives the same results as before.
Set and get scalars in decimate() and decimate_file().