Fix py_structs import errors across codebase

[kanishkaganguly]

When installing from source using python3 -m pip install ".[interactive]" and trying to run multical -h, there are import errors across multiple files.
This is running on Python3.8 inside a virtual environment.

Upon investigation, it would appear to be originating from the py_structs package, which seems to be using a different directory structure than what multical is expecting. This might be due to a change upstream, maybe.
This PR fixes all import errors and allows multical -h to run without crashing/errors. Whether calibration actually works is a different issue, and is under testing.

[oliver-batchelor]

I have committed a change to setup.py to change the dependency to py_structs < 1.0 (which is where the refactoring h to move the modules around happened).

If you're confident that it still works with these changes, I'm happy to commit it; really, it needs a basic test to check the functionality still works - but I'm not really using this for anything lately, so I've got zero time allocated to work on something like this.