Sandboxing PFFFT library

Builder: CMake

For testing: 
`cd build`, then `./pffft_sandboxed`

For debug:
`SAPI_VLOG_LEVEL=1 ./pffft_sandboxed --v=100
--sandbox2_danger_danger_permit_all_and_log <auxiliar file>`

CMake observations:
    * linking pffft and fftpack (which contains necessary functions for pffft)
    * set math library 

Sandboxed main observations:
    * containing two testing parts (fft / pffft benchmarks)
        ! current stage: fft - works :)
                        pffft - implemented
    * (Solved) pffft benchmark bug: "Sandbox not active"  
                        N = 64, status OK, pffft_transform generates error 
                        N > 64, status not OK
                        Problem on initialising sapi::StatusOr<PFFFT_Setup *> s; 
                        the memory that stays for s is not the same with the address passed
                        in pffft_transform function. 
                        (sapi :: v :: GenericPtr - to be changed)

                        Temporary solution: change the generated files to accept
                        uintptr_t instead of PFFFT_Setup

                        Solution: using "sapi :: v :: RemotePtr" instead of "sapi :: v :: GenericPtr"
                                  to access the memory of object s