Reconstruct library, C interface, call from GNU Radio reconstruct block

[samcrow]

Move the reconstruction into the GNU Radio process, called like a library from the work function

This should slightly improve performance, and allows Windows compatibility

• [x] Adjust interface of sparsdr_reconstruct library to make it more usable as a library
• [x] Make and document C interface to sparsdr_reconstruct
• [x] Update reconstruct block to use C interface to sparsdr_reconstruct library
• [x] Figure out build system to compile sparsdr_reconstruct library from Rust and then link it into the C++ gr-sparsdr library
• [ ] Test

[samcrow]

Note to self: When the reconstruct code is compiled as a static library, does it still link dynamically with FFTW? To avoid GPL infection, we don't want to statically link FFTW.

[samcrow]

It looks like the library libgnuradio-sparsdr.so has code that calls the FFTW functions but does not include the FFTW code itself, so the GPL situation should not change.