sci-electronics/ngspice: bump to version 27 and EAPI 6

sci-electronics/ngspice: bump to version 27 and EAPI 6

Major changes:
 * there is now a proper optional dependency on sci-libs/fftw,
   controlled by USE=fftw;
 * enabled OpenMP support, controlled by USE=openmp;
 * SENSdebug compiles again so it is now included in USE=debug;
 * the manual is only installed if USE=doc is set;
 * tests can now be run properly. At present two of them ("empty" and
   "resume", both in regression/misc/) fail, however it is only because
   of excessive output from the executable so it should be easy to fix.

Package-Manager: Portage-2.3.13, Repoman-2.3.3