The current notebook format requires sheets to explicitly reference every subcell that it wants to display:
<ipython-block>
<ipython-cell number="1" type="input"/>
<ipython-cell number="1" type="output"/>
<ipython-cell number="2" type="input"/>
</ipython-block>
We would like to keep that style as an option; occasionally you want to hide long outputs while keeping the input subcells or just see the outputs. However, this is somewhat inflexible, particularly when we start to allow extending the information available in the logs.
So by default, if an <ipython-cell> element appears without a type attribute, then it will display the whole cell with all of its subcells (or at least as many as the system is configured to display). If the type attribute is given, then only the named subcell is displayed.
This is a version-bumping change.
