Remove fft_save_wisdom.c

[David Dickinson]

The use of a c language file can complicate the build process slightly. It should be possible to use fftw's wisdom saving routines directly in Fortran with two methods

1. We use the default filename rather than allowing an arbitrary file. Simple but loses flexibility that we may want (but do we use this?).
2. Follow the approach here to provide a routine that fftw can use to read/write a character at a time.

Both methods are mentioned at http://www.fftw.org/fftw3_doc/Wisdom-of-Fortran_003f.html

[Peter Hill]

Can we use iso_c_binding to provide our own interfaces to the C routines? This obviously ties us to needing F2003

[David Dickinson]

We do use iso_c_binding already here, but to call our own c routines rather than calling the fft routine directly.

[David Dickinson]

See https://www.fftw.org/fftw3_doc/Wisdom-File-Export_002fImport-from-Fortran.html for a nicer approach. This is basically what we do in fft_work already except it seems like we might shadow the routines provided by fftw3 using our own versions (presumably to also support fftw 2)?

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[David Dickinson]

Resolved in PR #154