Support absolute units for any position, size or coordinate

[swhitty]

Any SVG position, size or coordinate can be specified with a unit (absolute or relative). This PR adds support for absolute units that can be explicated converted to pixels using predefined constants.:

• pixel
• inch
• centimeter
• millimeter
• point
• pica

<?xml version="1.0" encoding="utf-8"?>	 
<svg height="10cm" width="10cm" xmlns="http://www.w3.org/2000/svg">
    <rect width="10cm" height="10cm" fill="snow" />
    <polygon points="100,10 40,180 190,60 10,60 160,180" style="fill:lime;stroke:purple;stroke-width:5;fill-rule:evenodd;" />
</svg>

[codecov]

Codecov Report

Attention: Patch coverage is 96.15385% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.18%. Comparing base (b917e2a) to head (57a2676).

Files	Patch %	Lines
SwiftDraw/DOM.swift	96.42%	1 Missing ⚠️
SwiftDraw/Parser.XML.Scanner.swift	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   84.07%   84.18%   +0.11%     
==========================================
  Files          77       77              
  Lines        5437     5488      +51     
==========================================
+ Hits         4571     4620      +49     
- Misses        866      868       +2     

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