Printable G-code with correct extrusion calculation and misc improvements, more test models

[Spiritdude]

I documented all changes in new CHANGES file, here a copy:

2021/04/11: 0.8.5:

• adding -S shell_speed=..
• added Makefile and tests/Makefile and more test models

2021/04/06: 0.8.4:

• SVG output: supporting slicing to SVG, using "-o test.svg", very experimental
• intern: model.scale([x,y,z]) implemented
• cli: support for -M scale=s or -M scale=x,y,z
• intern: fixed vector.py normalize() and truediv() supporting STL with bad normals
• un/retract enabled again

2021/04/04: 0.8.3:

• cli & extending configs:
  • cool_fan_speed_{min/max}
  • cool_fan_full_layer=2 (starting fan after e.g. layer 2 is reached),
  • start_gcode and end_gcode and new config type 'str' as well
  • skirt_lines=3 only considered if skirt_layers != 0
  • gcode_comments = True|False (adding various useful info in Gcode)
  • random_pos = True|False: randomize position of model on the build-plate
• all the config included in Gcode
• intern: carrying over 'typ' ('type' in python ;-) of the extrusion in _raw_add_paths,
  to refine the retract/unretract, this allowed
  have output gcode more descriptive with ';TYPE:..',
• cli: support -l or --load=... to load more configs (e.g. per printer, or material)
• cli: support -Q <term> partial match
• output filament used at the end of slicing
• most un/retract disabled for now to confirm extrusion calculation working
• prints simple models like 10/20mm cube and xyzHollowCalibrationCubeV2
• finally was able to to run code without build & install, me newbie with python
• changed various from ... import ... to work locally
• created Makefile to support make build install

I also added comments in slicer.py for my own sake and other contributors to understand the code better. A few stale/inactive code I left inside for future changes and left old code available where I was unsure which approach is better.

Note: I'm rather new to Python so I struggled to find a way to code on the source files and tests it right away without build/install, so I changed all imports and alter sys.path in __init.py__ in case mandoline is called from installed location.