Improved init files and exceptions

[essweine]

This PR reorganizes some of the files into submodules and adds some imports to __init__ files.

The main changes are

• moving script engine related stuff to its own module
• consolidating lists of specs so they don't have be individually imported from the files where they're defined.

It also removes assert statements from the code, replacing them with exceptions where it makes sense.

There is also a bit of other cleanup of unused methods and useless imports.