Public API, part 1

[JohnLCaron]

1. Make the core objects (ucar.nc2., ucar.nc2.datatset.) immutable by adding builders.
2. Mark mutating methods deprecated.
3. Start to move static helpers to seperate class.
4. Move/port non-public classes to ucar.nc2.internal.

This should not be submitted until 5.2 branch is released.
In any case theres a lot more to do.

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This change is 

[JohnLCaron]

Ok, this is ready as soon as 5.2 is tagged.
Would be good to do asap so I can see the diffs for the next PR (coord system) which is pretty complicated. thanks

[lesserwhirls]

Awesome! 5.2 is now cut on 5.3.0-SNAPSHOT is on master. Would you mind rebasing?

[JohnLCaron]

done

…

On Wed, Oct 23, 2019 at 4:34 PM Sean Arms ***@***.***> wrote: Awesome! 5.2 is now cut on 5.3.0-SNAPSHOT is on master. Would you mind rebasing? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#137?email_source=notifications&email_token=AAEVZQCKHHFOOMHF6VH57VLQQDGRBA5CNFSM4I7GGWLKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOECDC4EA#issuecomment-545664528>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAEVZQHIHHY33NHTUFV56LTQQDGRBANCNFSM4I7GGWLA> .

[lesserwhirls]

Looks like just a spotless issue on the restored file. Once that's done we'll be good to go on this one.

[lesserwhirls]

I went ahead and applied spotless and pull this in over on #142. Thanks @JohnLCaron!