Create a front end to run a list of queries out of Main_queries

[pralitp]

@rplzzz, asked for the feature. While rgcam has the capabilities using the "query for a query" technique as described in the vignette it might be nice to formalize it with a front end method similar to how addScenario provides a front end for parsing batch queries and passing them to the runQuery / addQueryTable to do the heavy lift.

I'm thinking the front end parsing would take two arguments:

1. The path to the query file (Main_queries.xml)
2. An array of query names to query.

We can then generate a query for the query with something like:

paste0("doc('", query_file, "')//*[@title='", query_name, "']")

And for the gold star maybe try to detect if query_name is already an XQuery and if so run it unmodified (to provide a workaround for the multiple queries with same name problem).