spatial transcriptomics support

[jmvera255]

• spatial transcriptomic assay metadata template
  • each row would correspond to a slide, so collect slideID instead of libraryID
  • see HTAN for examples of additional attributes to include

[jmvera255]

first draft of spatial/imaging metadata template is now complete. I created a new context for spatial and imaging data types. I'm leaving this issue open for the time being until I review HTAN model