generalize and move mergeSampleMetadata

[d-callan]

there is a cool helper i made here called mergeSampleMetadata which takes two SampleMetadata objects and merges them and returns a single SampleMetadata object. its worth considering if we can generalize it and move it to veupathUtils. not urgent id think though.

[d-callan]

also mergeSampleMetadata wont work for metadata from different branches of the tree. that could be part of the generalization effort, or a separate ticket if we decide not to generalize.