Replaced all convenience vector lists ( UP_VEC = [0,0,1], etc)   with tuples:  (UP_VEC = (0,0,1)  ) so  they're immutable and can be used as hash keys
Added BoundingBox class. Changed split_body_planar to me more robust and use BoundingBox.
Changed tests to use new split_body_planar()
Added add_wall_thickness arg to BoundingBox.split_planar() so new bounding boxes can add area around them for mold-making purposes.  Default behavior is no-op
Added note on argument requirements & list size assertion in fillet_2d()
Fixes #20, so that OpenSCAD’s use() and include() functions still work, even if you don’t import SolidPython as ‘form solid import *’
Fixes #21.  SolidPython now passes any extra arguments on to OpenSCAD.
Added tests for include, use, and passing extra arguments to OpenSCAD.  Fixed a bug that meant that include() has never (!) worked.
Merge branch 'feature/Accept_extra_args_to_SCAD_module_GH_#21' into develop
Merge branch 'feature/scad_import_without_python_star_import' into develop

Conflicts:
	solid/solidpython.py
committing merge?  Thought I just did this.
Merge branch 'develop'