Svgelements

* Speedup for Cubic Length if Scipy is installed. (Thanks to abey79)
* arc_angle() function now accepts ccw kwargs. The default is still whether angle a0 is greater than angle a1. However, in some cases the it's correct to insist on a strict chirality.
* Structural Parsing of style blocks within def blocks will pass that element to the main parsing routine. So that <defs><style/><defs> code isn't omitted. This lead to a bug in some files.
* Appending two different CSS classes doesn't fail the assignment. These would strictly concatenate rather than include the `;` token between them.

* Coord values without points raise ValueErrors. Path("sn") will throw an error rather than create a faulty object.
* Various dunder methods will return NotImplemented explicitly.
* Adding a Path with something that cannot add be added to it, will register a TypeError rather than recurse an infinite loop.
* Reify performed on a shape with a stroke width will be scaled up by a factor of the `sqrt(abs(det))` of the matrix.
* Validate Subpaths will not be applied to position `-1` as could happen in Path.extend() on an empty path.
* scale values for round shapes during reification is done as the absolute value. This is to correct the inverted sweep issue.
* Roundshapes have an added `_ramanujan_length` value.
* Subpaths now accept slices as part of getitem.
* svg_structure_parse now parses event start-ns and saves the namespaces used in the values dict.
* Default colors for fill are "black" and stroke is "none" rather than automatically setting them to the `currentcolor`, these values match the svg spec.

Preserves Relative and Smooth values as initially set in the path. These can be overridden by setting `relative=True`, `relative=False`, `smooth=True`, `smooth=False`.

Rebuilt the core SVG path parser. This was required to pass W3C Path17 and Path20 tests. The code now correctly parses svg arc flags as either 0 or 1 rather than a float. And will stop parsing paths if unknown characters are found within the data.

Corrects an issue with rotated SVGText bounds. And an issue where the bias in validation could produce wrong results for .reverse() call.

Corrects the PathSegment bounding boxes for Arc, QuadraticBezierCurve, and CubicBezierCurve.

Lengths for almost collinear quads could be outside the error range but still so close as to divide by zero or hit a domain value error.