named clickSelects

[tdhock]

clickSelects is already a character vector but the names are ignored.
If names are present we could interpret it to mean names=selectors, values=column names.

OPTIMIZATION: for the prostateLasso test, it would be great to be able
to specify clickSelects=c(arclength="arclength.click") even if there
is no column named arclength in the data -- this would mean to update
the selector called "arclength" with the value in column
"arclength.click" -- less disk space, and faster compiling.