#914 specify level orientation

[bena-nasa]

This addresses #914
The GCHP users requested the ability to flip the native output relative to the input. It was agreed that for now we will assume History always gets fields "down" in that level 1 is the top of the atmosphere. This is true in GEOS and in GCHP they adjust the inputs to History so the is the case. However, they were requesting that it would be nice if you could get output from History that conforms to the GCHP convention that level 1 is the surface so it would be less confusing for users.
This PR adds a 'positive' option to the history collection. By default it is 'down' and the only other allowed option is 'up'. If you specify any "levels" either to select model levels to regrid to pressure, height etc ... this is ignored, so only used if you are specifying full native level output. We assume all 3D fields coming to history will be 'down' as in GEOS and so by default nothing is done. If you specify 'up' the positive attribute in the output is now 'up' and the levels will be flipped relative to what came into History.

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Trivial change (affects only documentation or cleanup)

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• I have tested this change with a run of GEOSgcm (if non-trivial)
• I have added one of the required labels (0 diff, 0 diff trivial, 0 diff structural, non 0-diff)
• I have updated the CHANGELOG.md accordingly following the style of Keep a Changelog