Updates and bug fixes

[Stoops-ML]

• Bumps to v2.1.0
• Updates readme
• Sequence -> TimeIntervalCollection
• Add validations on many properties
• Correct usage of Position, PositionListandPositionListOfList` (according to the czml-writer wiki)
• Type classes cannot be initialised to None
• Fix usage of SolidColorMaterial in Ellipsoid
• Fix usage of 'solidColorinMaterial`
• Fix usage of 'polyOutlineinMaterial`
• All properties can be TimeIntervalCollection
• Add Cartesian3VelocityValue type
• Expand testing
• ReferenceValue.string -> ReferenceValue.value
• RgbafValue and RgbaValue checks time values

[codecov]

Codecov Report

Attention: Patch coverage is 99.65577% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.53%. Comparing base (993f614) to head (406ca83).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/czml3/properties.py	99.52%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
+ Coverage   99.44%   99.53%   +0.08%     
==========================================
  Files          11       11              
  Lines         898     1064     +166     
==========================================
+ Hits          893     1059     +166     
  Misses          5        5              

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[Stoops-ML]

@astrojuanlu bump