Speed up demodulation in demod task

[MatthieuDartiailh]

Issue raised by @leghtas in Exopy/exopy#86:
When data is acquired from an acquisition card, we extract I and Q by multiplying the raw traces by sin and cos, and then taking the mean. This operation, can, in some cases, be the limiting factor for how fast we can take data. Maybe there are tricks we can play in the demodulation task to make it faster?

I have not yet taken the time to look in details to the changes to demodulation in #8 and I need to do so before answering your question but we can probably come up with something. However I probably won't have time to look at it before leaving next Friday.