Setigen

Update requirements and installation documentation.

* Use sigma clipping for better noise estimation
* Support usage of arrays / floats for path, t_profile, and bp_profile in Frame.add_signal
* Optimize signal injection by removing expensive / superfluous calculations
* Added a lot more documentation, covering all substantial functions
* Update Sphinx docs to point to docstrings: https://setigen.readthedocs.io/en/master/setigen.html

Focus signal generation around the Frame object, with the following updates:
* Load and save data from arrays or fil/h5 files
* Add Gaussian noise throughout the frame, either by user-provided distribution parameter or by sampling from pre-loaded parameters drawn from real GBT observations
* Add custom signals, from a simple, constant-intensity line to complex, modulated signals through a combination of built-in or user-defined callbacks
* Integration with astropy units, so that frame parameters and synthetic signals have physical meaning.
* And more instance methods that simplify calculating basic statistics, SNR to intensity, etc