Setigen

Support basic doppler smearing functionality within `Frame`. Normalization within `integrate`.

* Add `frame.get_slice()` method, which constructs a new Frame based on index bounds
* Add `frame.copy()` method, which additionally handles Waterfall() object copying
* Add `frame.integrate()` method, to easily construct spectra or time series data
* Add `frame.from_backend_params()`, to construct an empty frame based on data reduction parameters
* Add `frame.plot()` and `frame.bl_plot() `as aliases for `render()` and `bl_render()`
* Add `stg.array()` to cast either a Frame or a numpy array to a numpy array
* Change `end_index` to `stop_index`
* Bugfix `int_factor` and support `data` parameter in `from_backend_params`
* Add `stg.dedrift()` function

- Changing `raw_file_stem` to `output_file_stem` in `RawVoltageBackend.record()`, simply to be more descriptive
- Support injecting raw voltage signals in existing data; i.e. starting a raw voltage generation pipeline with existing RAW data via an `input_file_stem` parameter
- Switch filterbank/quantization object data cache standard from one cache to cover all (antenna, polarization) pairs to a separate object and associated cache for each (antenna, polarization) pair
- Updated docs

Include voltage header asset with pip installations

Use raw external image links in README.md, so that images show in PyPI description.

Rename default branch to `main`, edit README image links.