Setigen

Support frequencies in ascending order in Frame. Clean up get_frequency and get_index methods. Edit split_utils to also generalize to ascending frequency files.

* Add sinc squared as an `f_profile` (`sinc2_f_profile`). Using the `trunc` parameter controls whether to stop the signal after the first zero-crossing.
* Add a new `path` for simple RFI-like signals (`simple_rfi_path`). Can control width of variance, and whether the signal varies about a straight line path or accumulates frequency offsets as a random walk.
* Update docs accordingly.

Patch setup.py to include package data in installation.

Make the default noise background generation follow chi-squared distribution, appropriate for radiometer noise. Noise is calculated using data frame resolutions, and scaled up using a mean value. This change affects `frame.add_noise()` and `frame.add_noise_from_obs()`. Using a normal distribution instead for noise is still possible, using the `noise_type` argument, but for most (high spectral resolution) cases, using chi-squared noise is the best option.

* Use linspace instead of arange for better precision
* Fix get_frequency method to support signals that go out of frame in add_constant_intensity

Make sure dependencies are up to date -> blimpy 2.0.0