Comments and tests for Seq module

[kevmal]

Closes #329, Closes #330

• Add xml comments to Seq, #281
• Fix Seq.weightedMean #329
• Fix Seq.stats #330
• Add tests for Seq module

[Required] please make sure you checked that

• [x] The project builds without problems on your machine

[Optional]

• [x] Added unit tests regarding the added features