fourier. Add one overload to make_gf_from_fourier API.

- To be able to give explicitely the number of point.
- two functions : if the number of points is not given, take as before.
- updated the doc.
Fix #100.

- The workaround for align_t in C++14 mode has been fixed in recent
  version of clang.
- Does not apply to OS X (it uses libc++).