Visualizations: Handle multi value vars

[d-callan]

They'll be represented as json strings, which we need to break into multiple rows. Think we should also have isMultiValued in the var metadata map

[d-callan]

think we need to decide if there are any special constraints or features required which are specific to particular vizs before this can be costed.

[danicahelb]

I think the current functionality is that any term that has at least 1 multivalued response cannot be selected to be used in any viz tools.

Perhaps we should consider just removing multivalued responses and plotting all uni-valued responses for that term. This will work well for terms that have only a handful of multivalued responses.

There is still an open question about how terms with majority multivalued responses should be handled

[danicahelb]

Related to VEuPathDB/EdaDataService#64