Parse JDX matadata into `@data`

[GegznaV]

This originally was a comment #23 (comment) by @cbeleites. But it is worth a separate issue:

---

For some of the other import filters, I use a keys2data argument to specify which metadata should be turned into extra data columns. It is actually used to index such metadata lists: list_jdx[[2]][keys2data]

I suggest using the same here.

• keys2data = FALSE no key used
• keys2data = TRUE all available keys (useful to get an overview over "unknown" files)
• keys2data = c("$FILENAME", "$SCAN") -> you get the idea

---