* Converted PositionUnits to Property

* Created `ConvertDistance` method & friends
(Thanks Wybren van den Akker for helping with  math)

* Added `ArgumentOutOfRangeException`s to internal code (could theoretically be accessed if users use ASMREFs)

* Changed the order of upgrading. `m_Path` has to be done first, `CinemachineSplineDolly` needs it before its `PositionUnits` is set.

* Added Unit Tests for my changes

* Added XML Documentation to new Conversion methods.

* `PositionUnits_DoesNotChangeCameraPosition_WhenPositionUnitsSame()` - Added additional asserts, not very important because as the code is now even just one should be enough. But still.

* Fixed small error made in XML comments

* Moved my positionUnitsBackingfield change tracking from OnValidate to the custom editor.

…lti-select