Allow setting FFTW_EXHAUSTIVE

[johnomotani]

May sometimes make FFTs faster than the previously available FFTW_MEASURE setting.

Also uses a BOUT_ENUM_CLASS() for the option, which should help ensure that that doesn't get broken in future :-)

Unfortunately the macros don't work inside the bout::fft namespace, I think due to FFT_FLAG needing to be passed as a template parameter in various places. I don't understand why that didn't work though.

Performance - FFTs seemed to be faster with the exhaustive flag on Marconi, but I haven't checked with BOUT++ yet. Having the option for the exhaustive flag can't hurt anyway.

Todo:

• document new option in manual

[johnomotani]

The new option needs documenting in the manual. I'll do that before we merge.

[ZedThree]

Do we want to deprecate the fft_measure input? There's a v5 upgrader for renaming input parameters, it could probably also change the values at the same time.

The issue with putting a BOUT_ENUM_CLASS declaration in a namespace is that the specialisation of Options::as needs to be in the same namespace as Options. I managed to get it to work like this:

namespace bout {
enum class FFT_FLAG;
FFT_FLAG BOUT_MAKE_FROMSTRING_NAME(FFT_FLAG)(const std::string& s);
}

BOUT_ENUM_CLASS(bout::FFT_FLAG, estimate, measure);

but I can't see how to make it work in the macro without knowing the namespace name, i.e. BOUT_ENUM_CLASS(bout, FFT_FLAG, ...) can work.

[ZedThree]

BTW, the reason I worked out why the namespace wasn't working was I tried compiling with clang, which gave me this error:

/home/peter/Codes/BOUT-dev/working/test_enum.cxx:8:1: error: function template specialization of 'as' not in class 'Options' or an enclosing namespace
BOUT_ENUM_CLASS(FFT_FLAG, estimate, measure);
^
/home/peter/Codes/BOUT-dev/include/bout/bout_enum_class.hxx:96:36: note: expanded from macro 'BOUT_ENUM_CLASS'
template <> inline auto ::Options::as<enumname>(const enumname&) const -> enumname { \
                                   ^
/home/peter/Codes/BOUT-dev/include/options.hxx:355:5: note: explicitly specialized declaration is here
  T as(const T& UNUSED(similar_to) = {}) const {
    ^
1 error generated.

whereas gcc gave me

In file included from /home/peter/Codes/BOUT-dev/working/test_enum.cxx:1:
/home/peter/Codes/BOUT-dev/working/test_enum.cxx:8:17: error: ‘FFT_FLAG’ does not name a type
    8 | BOUT_ENUM_CLASS(FFT_FLAG, estimate, measure);
      |                 ^~~~~~~~
/home/peter/Codes/BOUT-dev/working/test_enum.cxx:8:17: error: ‘FFT_FLAG’ does not name a type
    8 | BOUT_ENUM_CLASS(FFT_FLAG, estimate, measure);
      |                 ^~~~~~~~

Not quite as helpful as clang!