Additional time-travel support

[eddelbuettel]

This PR adds high-level support for timestamps at the tiledb_array level. A simple illustration is included as well (and the basis of added unit tests): we open an array, write three rows at a recording timestamp, sleep a little and write three more rows. The expected reads before first, after first, before second and after second time stamp are the expected 0, 3, 3, and 6 rows.

$ Rscript inst/examples/quickstart_sparse_timetravel.R
Array already exists, removing to create new one.
Writing first record at 2020-12-28 17:35:05.074403 
(...sleeping 1s ...)
Writing second record at 2020-12-28 17:35:06.104814 

Reading everything:
  rows cols  a
1    1    1  1
2    2    3  3
3    2    4  2
4    6    7 22
5    8    5 11
6    9    8 33

Opening / reading 0.5s before first tstamp
[1] rows cols a   
<0 rows> (or 0-length row.names)
Opening / reading 0.5s after first tstamp
  rows cols a
1    1    1 1
2    2    3 3
3    2    4 2
Opening / reading 0.5s before second tstamp
  rows cols a
1    1    1 1
2    2    3 3
3    2    4 2
Opening / reading 0.5s after second tstamp
  rows cols  a
1    1    1  1
2    2    3  3
3    2    4  2
4    6    7 22
5    8    5 11
6    9    8 33
$ 

[aaronwolen]

Two small comments but LGTM!

[aaronwolen]

Small typo: datimetime

[eddelbuettel]

And we all sing now :"dah dah dah dahdatime".

Nice catch -- fixed.

[aaronwolen]

Maybe mention it must be POSIXct?

[eddelbuettel]

Done too -- both fixes in b714f92