Remove normalization from stochastic background

Proposed by @qbaghi.

We currently normalize the amplitude (strain) of each pixel source by the number of pixels.

# Compute the generator for the pixel
pixel_generator = lambda fs, size: self.skymap[pixel] * self.generator(fs, size) / self.npix

This is unconventional, and there's probably a missing square root. We should leave it out, and according to the documentation, leave any normalization to the user.