- Get rid of helper functions. It is not that much more verbose to
create signals with `argus.FloatSignal(...)` than
`argus.signal(..., dtype=argus.dtype.float64`).
- Make the package hierarchy flat: everything is under `argus`. If this
is an issue, it can be changed in the future.
- Add type hints for interval types.
All methods that need to perform interpolation of some sort need an
explicit interpolation method. In Rust, this manifests as a generic
parameter, while in Python, this is a string parameter.
