Modules export only intended public elements

[pp-mo]

• Consider all public names
• Avoid sub-module imports
  • some existing ones may be wanted : e.g. iris.cube, iris.coords
  • most are "accidental" and should be avoided : like Import of 'iris.analysis' provides 'iris.analysis.interpolate' again. #2121
• Don't export the module's own imports (sys, six, numpy, netcdf4 ...)
  • can be done with widespread renames adding underscores
  • Alternatively, much less fiddly to use "all", but it's not clear how universally respected the 'all' convention is ?
  • see : Userguide section and whitepaper on user change management. #1999 (comment)

[pelson]

The only tangible benefit we can come up with for this is to avoid things like iris.logging where logging is imported in iris/__init__.py.

I notice that packages like matplotlib haven't solved this one either: >>> plt.np is completely valid...

For me, this doesn't feel like a huge win, nor particularly urgent, so I'll close this. Would probably merge a PR that addressed this though...