3.24.15 save phase

Saves the current phase information in Xronos window format. With a single entry on the phase-filter stack, one file is written. With multiple entries the whole stack is saved: each entry has its own epoch and period so they cannot be merged into one window file, and the files are written with the same _p<N> suffix used by extract spectrum per=phase — save phase base.wnd writes base_p1.wnd, base_p2.wnd, and so on. The saved set can be reloaded later with filter phase file.