Add pyfftw sdp

[NimaSarajpoor]

This is to address PR 3 described in #1118 (comment). Have copied the corresponding notes below:

1. Add _PYFFTW_SLIDING_DOT_PRODUCT to sdp.py
2. Add unit tests that demonstrate that _PYFFTW_SLIDING_DOT_PRODUCT matches the results of naive_rolling_window_dot_product
3. Handle the test.sh check_fftw_pyfftw stuff here too