Extend query_object for var char columns

[eddelbuettel]

This PR adds some more polish to the recently added tiledb_query S4 object letting it also operate on variable length character columns -- which in turn makes a number of the examples more concise and somewhat more natural for R users.

[aaronwolen]

Having a look now.

I noticed a few different argument names are used for size offsets and size data:

• sizeoffsets, szoff, szoffset
• sizedata, szdat

[eddelbuettel]

Thanks. They should all be internal but easy to rename.

The key point of the PR is that a variable

R> var <- c("The", "quick", "brown", "fox")
R> var
[1] "The"   "quick" "brown" "fox"  
R>

can now be used rather than a collated single string and offsets vector which is a little more foreign to R users (and still used in the unit tests etc).

[Shelnutt2]

Very nice usability improvement

[eddelbuettel]

Thanks @seth. There may be more to do still on that front but we "got to flip those stones one at a time" so to speak.

I'll look into the suggestion by @aaronwolen and standardize the variable names some more and merge then.

[aaronwolen]

Everything worked for me. This is indeed a nice convenience R users will appreciate!

[eddelbuettel]

@aaronwolen If you have a second glance at the commit 2942e16 I just added which should clean things up a bit more.

[aaronwolen]

LGTM—I like the longer arg names 👍