Provide a function to export as EML XML? about dataspice

[cboettig]

We talked about doing this but I don't think we ever got around to implementing it(?) IIRC dataspice currently implements EML->dataspice but not the reverse. @annakrystalli would know better than I though

[amoeba]

I have a not-perfect but pretty-good dataspice->EML conversion set up going. Workflow is:

spice <- read_spice(...)
eml_doc <- as_eml(spice) # Followed by an EML::write_eml if you want XML

This produces a probably invalid EML record because of terms that don't easily crosswalk to EML without me writing tons of parsing logic. The biggest issue is that schema.org/distribution and schema.org/variableMeasured don't map well to EML because schema.org has us enter variables at the dataset level where EML associates variables with dataTable elements. To make this less painful, I've provided a warning indicating this and a a crosswalk_variables function to start the user off building an EML attributes table.