Cleaned up top-level namespace

[philippjfr]

As suggested in #2202 this PR cleans up the top-level namespace, removing the following items:

Element2D, Element3D, Dimensioned, AttrTree, Overlayable, StoreOptions, UniformNdMapping, ViewableElement, Overlayable, CompositeOverlay

[philippjfr]

Pared this PR back a bit to make it mergeable in the near term, Element, NdMapping and BoundingBox are therefore still exposed.

[philippjfr]

Here's the full list of items removed from the top level namespace:

{'AttrTree',
 'CompositeOverlay',
 'Dimensioned',
 'Element2D',
 'Element3D',
 'Overlayable',
 'Tabular',
 'UniformNdMapping',
 'ViewableElement',
 'absolute_import',
 'code',
 'elements_list',
 'f',
 'filename',
 'os',
 'print_function',
 'pydoc',
 'rcfile',
 'warnings'}

[philippjfr]

Going to merge this, @jlstevens gave assent in the linked issue and I have undone the removal of Element that was mentioned as potentially problematic there.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.