concise package import

[shimwell]

This PR fixes issue #14 by adding a file import to the init

examples and tests have been updated to use

import assembly_mesh_plugin

instead of

import assembly_mesh_plugin.plugin

[shimwell]

now passing lint tests, this one is ready for review

[jmwright]

@shimwell Do you have a feel for how many scripts are using this plugin now that will be broken by this import change? It's not hard to fix, but it's always a shock when you upgrade or reinstall a plugin and its behavior has changed like this.

[shimwell]

I've only seen my own scripts which I've fixed in fusion-energy/cad_to_dagmc#128

Not seen any other use. There might be some but I've not seen it.

Perhaps we use the rust style versioning to indicate breaking API changes and release v0.2.0 after this PR

Also I'm not sure if it actually does break the current way of using it.

[jmwright]

Thanks @shimwell !