Make fabsim a regular Python package

[matt-graham]

Part of addressing #250

Makes fabsim directory a regular Python package by adding empty __init__.py files to fabsim directory and subdirectories containing Python modules we wish to be able to import (assumed all excpet bin directory).

As a regular Python package rather than implicit namespace package, fabsim directory will be identified as importable as soon as first encountered on the search path rather than after searching all paths

Creation and loading of a regular package can take place immediately when it is located along the path. With namespace packages, all entries in the path must be scanned before the package is created. (https://peps.python.org/pep-0420/#discussion)

For now I have just left all the __init__.py files empty. These could be used to expose names at the subpackage level (allowing importing names directly from fabsim.base or fabsim.lib for example), but to keep the changes minimal I've not done this here.

[matt-graham]

This change should not require any modifications to code elsewhere in the codebase or plugins. I've tested running FabDummy and FabNESO tasks locally after these changes and both run without any issue.

[djgroen]

Fairly straightforward change. Looks all good to me!