MAINT: Use "shape" for variables derived from array.shape (not array.size)
ENH: Speed up fftconvolve by using 5-smooth length to pad FFT
instead of padding to the next power of 2.  FFTPACK can handle multiples and powers of 3 and 5 just as efficiently as powers of 2.
TST: signal: add some tests for fftconvolve padding
Merge pull request #3144 from endolith/smooth_fft

ENH: Speed up fftconvolve by using 5-smooth length to pad FFT

The speed of fftconvolve can be improved by choosing the padding more
carefully --- it's not necessary to use exactly power of 2, since
FFTPACK does handle also composite numbers of 2, 3, and 5.