* Move Gridlines from Chart to Axis
This could, I hope, be my last major change to Chart for a while. When I first noticed this problem, I thought it would be a breaking change. However, although this change establishes some deprecations, I don't think it breaks anything. Major and minor gridlines had only been settable by the Chart constructor. This PR moves them where they belong, to Axis (eexisting Chart constructor code will still work). This allows them to be specified from both X and Y axis.
Chart is now entirely covered except for 2 statements, one deprecated and one that I just can't figure out. 99.71% for Charts, 88.96% overall. All references to the Chart directory in Phpstan baseline are eliminated.
* Minor Fixes, Unit Tests
Line style color type should default to null not prstClr.
Chart X-axis and Y-axis should alway be Axis, never null.
Add some unit tests.
* More Tests, Some Improvements
Make it easier to change line styles, adding an alternate method besides a setter function with at least a dozen parameters.
Unit testing now results in 100% coverage for Axis and Properties. All the properties in methods in Gridlines were more or less duplicated in Axis, and these duplications are moved to the common ancestor Properties. So, there isn't anything left in Gridlines. PhpSpreadsheet Chart is now over 85% covered (it was below 35% until recently).
Properties are in many cases set to default to null/null-string, rather than the default values they receive from Excel, and are not written to Xml if unchanged. This is consistent with how Excel behaves. A new property `crossBetween` is added to Axis, and, with support for that added to Xlsx Reader and Writer, some minor Sample peculiarities are corrected, in particular, the charts were sometimes slightly truncated on the left and right edges.
oleibman