SVG export enhancements

[lorenzncode]

Add option to set scaling based on model dimension instead of autoscale.
Change default projection to match cq-editor.
Add option and set default "up" direction so it does not flip with changes to projectionDir.
Remove margin from autoscale calculation when autoscale enabled. This allows margins to be edited without affecting scale.

Resolves #729, resolves #730.

[codecov]

Codecov Report

Merging #1277 (ab7c6fe) into master (9c519e6) will decrease coverage by 0.07%.
The diff coverage is 66.66%.

❗ Current head ab7c6fe differs from pull request most recent head e18197d. Consider uploading reports for the commit e18197d to get more accurate results

@@            Coverage Diff             @@
##           master    #1277      +/-   ##
==========================================
- Coverage   94.16%   94.10%   -0.07%     
==========================================
  Files          26       26              
  Lines        5451     5459       +8     
  Branches      925      927       +2     
==========================================
+ Hits         5133     5137       +4     
- Misses        188      191       +3     
- Partials      130      131       +1     

Impacted Files	Coverage Δ
cadquery/occ_impl/exporters/svg.py	93.54% <66.66%> (-3.01%)	⬇️

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[jmwright]

It seems like this PR is also related to #1282 and #487

[Jopie01]

Currently it is not supported to let the exporter decide the size of the image. you have set a height or width.

It would be nice to let the exporter decide the height and width based on the boundingbox and the margin. All based on a unitscale of 1 instead of the 0.75.