Identify avoidable use of scipy at runtime

[marcusmueller]

#1545 pointed out that there's scipy used at test-time, but there's also a lot of scipy used in run-time tools and examples that we deliver. (gr_plot_*, filter design tool, a lot of the digital, dtv and filter examples, and more).

In many cases, it's a matter of getting fftpack, which is a) part of numpy and b) could probably be replaced with our very own gr::fft code.

We should identify these cases and reduce the usage of optional external dependencies.

[marcusmueller]

This seems solved.