pycuampcor deramp improvement

[lijun99]

This PR improves the deramp procedure in pycuampcor/dense_offset to address the issue of (linear) phase ramp along azimuth due to the Doppler Centroids.

1. fix the pixelIdx error (thanks to @bhawkins)
2. use double precision for phase correction. Using float seems to introduce spectral noises which breaks the band-limited structure and therefore lowers the correlation.
3. add derampAxis parameter for options to deramp along certain direction, 0=azimuth 1=range and 2=both axes(default). In cfg, use deramp_axis = "azimuth", "range", or None to set this parameter.
4. move cuDeramp from oversampler to chunk processor; making the code more readable