MAINT: Use lazy_loader for lazy loading of submodules

[SimonHeybrock]

This is @MridulS change from #595 + tox -e deps.

Fixes #594.

[nvaytet]

I think Mantid tests are failing because you also need to add lazy_loader to the .buildconfig file?

[SimonHeybrock]

@MridulS Docs builds are failing, can you have a look?

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[9], line 2
      1 wavelength_graph_monitor = {
----> 2     **scn.conversion.graph.beamline.beamline(scatter=False),
      3     **scn.conversion.graph.tof.elastic_wavelength('tof'),
      4 }

File ~/micromamba/envs/ubuntu-latest/lib/python3.10/site-packages/lazy_loader/__init__.py:94, in attach.<locals>.__getattr__(name)
     92     return attr
     93 else:
---> 94     raise AttributeError(f"No {package_name} attribute {name}")

AttributeError: No scippneutron attribute conversion

[SimonHeybrock]

Thanks @MridulS, latest changes LGTM.