suzerain: make one-off methods public

I'd not declared p3dfft_get_precision, p3dfft_using_stride1,
or p3dfft_fftw_rigor to be PUBLIC and consequently GCC/4.8
stopped emitting symbols for them.

As I'm using none of the ghost cell support, I'm no longer
dumping out those symbols.

Added FIXME re: get_dims name.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44138 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: get_dims -> p3dfft_get_dims

Renames the publicly exported symbol.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44139 0bd85f0b-c151-43b9-a1e9-27231104551f