Lazy read of attributes

[edhartnett]

Part of #857.
Part of #489.
Fixes #1025.

In this PR I change attribute reading to lazy. No attributes are read at open. Whenever the user gets/puts/inqs an attribute for a var or group, then all atts for that var or group are read. If atts in a var/group are never touched by the user, they are never read.

This helps a lot with slow open times for files with many attributes.

[DennisHeimbigner]

Did this get merged? Why is it closed?

[edhartnett]

This is in the queue. When #1023 gets merged this will be next.

[DennisHeimbigner]

nc4_set_default_vars needs to be restored

[edhartnett]

nc4_set_default_vars does not have a place any more. It used to call the vara versions of the function. But now there is no vara version of the function. The vara calls also call the vars functions.

So to see the difference between breaking up the write/read into varas instead of vars, you can call NCDEFAULT_vars, but it will still end up calling the vars functions under the hood.

[DennisHeimbigner]

Then I will put back NC4_get/put_vara and NC4_get/put_vars

[edhartnett]

OK, you may wish to simply unwind the most recent PR that was merged, which included those changes.

Be advised that although I didn't enter tickets for it, I believe there were some additional issues in that version of the vars functions. Handling of zero counts did not work until I made some changes to the vars code. I suspect there are simply more and better tests for the vara code.

I will take down this PR while all this is resolved.

[edhartnett]

OK, Dennis and I have resolved the request for changes, and this PR is ready to merge.

[WardF]

@DennisHeimbigner can you remove your request for changes? Thanks!