suzerain: repair parallel FFT selection

When !fftwdef, this used to cause a boost::any_cast failure.
The new logic should avoid that by better relying on bool_switch.

Also, avoids the ABI issue wherein the header changed depending
on configure-time library availability.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43576 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: correct comment

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43577 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: permit null {restart,stats}def

This is admittedly not pretty.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43578 0bd85f0b-c151-43b9-a1e9-27231104551f