Improved periodogram calculation plus more options in per

Implementations of three types of periodograms are now available: Lomb-Scargle
using the fast method of Press and Rybicki; Generalized Lomb-Scargle from
Zechmeister and Kürster (2009); Bayesian version of the periodogram as in
Bretthorst (2001), Gregory (2005).
Use of each algorithm is controled by optional flags to the 'per' command.
Fortran extension for BLS plus makefile; correct import is tested