Warnings about future array shapes

[bhazelton]

Should we start warning on file read about future array shapes? We could add an option to read to convert to the future shapes on read to avoid the warning.

This also applies to the future phasing -- we should probably follow the same plan. We have an issue to have deprecation warnings for accessing the old phasing attributes (#1094), but we need to decide when those are raised.

[bhazelton]

We agreed to make this change. We'll add the option to read to silence the warning. We should set the keyword to None (to detect that it wasn't set), with default behavior of the current shapes. Then we can change the default behavior to be the future shapes in a future version (before actually removing support for current shapes which will happen in 3.0).