Fix node.labs not working for alphanumeric alpha.ids

[jrgant]

qd_dag needs to check for pre-existing labels in nodes with alphanumeric node.ids because these nodes are assigned a label at graph creation (to subscript the number)

For instance:

edges <- c("A1 -> Y") labs <- c("A1" = "Exposure", "Y" = "Outcome" qd_dag(edges, node.labs = labs) %>% render_graph()
will update the Y node's label but not A1's.

[jrgant]

Related issue: qd_save() fails with a SWIG that has nodes defined by alphanumeric IDs. Error probably comes from rsvg_pdf()

[jrgant]

Probably makes sense to turn off subscripting by default and allow the user to declare that they want an alphanumeric ID subscripted.

[jrgant]

Subscripting is now turned off by default.

Add an example to show how to handle time-varying node labels.