Search for peaks over a wider range

[argilo]

After the removal of scan plot data decimation in #939, some peaks are displayed with incorrect amplitude:

This is because the code that searches for the peak amplitude in the bins near the quantized frequency does not search far enough. It only looks from 5 bins down (-4000 Hz) to 4 bins up (+3200 Hz), even though the original peak frequency may be up to 10000/2 = 5000 Hz away.

Here I've solved the problem by calculating the required search radius based on the quantization and search_step values. I also fixed the off-by-one in the upper limit. (Python ranges do not include the end value.)